I've been using the sandstone black variant of the OnePlus One for a little more than three months now. The OnePlus One has an incredible value for money, and it's feature set is at par, if not better than most flagships of 2014. However, there are concerns for the future of OnePlus because of it's legal issues with Cyanogen. If you are on the fence on whether to get one, read on to get an idea about the long-term user experience.

Design

Look and feel

The One has a stunning, yet minimalistic design. The metal band surrounding the screen gives it a very premium feel, and the texture of the back panel is very pleasant. It's sandstone finish is one of the better examples of aesthetically appealing, yet comfortable to use back panel designs. 
The only downside that I've run into is that replacement back panels are unavailable. This won't be a problem for most, but after having dropped it from ear height twice, I've managed to chip off a bit of the textured coating around the corner of the back panel

Button placement

Placement of volume and power keys is quite intuitive, and are easy to reach. The capacitive buttons, however are a bit troublesome. Because they're closer to the screen compared to those on other devices I've used, I found myself frequently touching them by accident. Over time, I've become more accustomed to this, but it can be a bit of a nuisance at first.

Screen

I was concerned about the raised display, as it is more exposed and prone to damage. However, after having suffered at my clumsy hands for so long, it is remarkable that there isn't a single scratch to be seen on the screen. I've put it in the same pocket as keys and coins (consistently), dropped it face-down (multiple times), and even dropped a 2.5 inch laptop HDD on it (it was the shock of my life). I think it is safe to conclude that you won't have to worry about scratches. The screen is resistant to smudges, but it takes a lot of effort to clean smudges off once they appear. I spent a good fifteen minutes polishing the screen to make it look so shiny in the image above.

Hardware

Display

Up until I bought the One, I used a Galaxy S3, which had a 720p AMOLED display. In comparison, the OnePlus One features a 5.5” 1080p full HD JDI screen with LTPS. The One's display is nothing to complain about, but it simply can not match the contrast and vibrance of AMOLED displays. If you watch a lot of videos on your phone, you might want to consider the Nexus 6 for it's display.
There is, however a big benefit to not having AMOLED. I no longer have to worry about burn-in. My S3's screen now has a yellow tint because I themed my home screen blue, but I don't have to worry about such things on the One.

Speakers and microphones

While most manufacturers are still confounded by the challenge of placing the speakers properly, the people at OnePlus realized that putting it on the backside of the phone is a dumb idea.

The speakers are loud, and have incredible clarity even at high volumes. There is a trade-off though. The magnets used in the One are strong enough to wipe your credit cards if you put them in the same pocket, though not strong enough to hold bottle caps like the iPhone 6 Plus.
If you still feel that you need MOAR POWAH!!!!, abhi08638 over at XDA Forums has you covered with his OnePlus One Audio Mods Collection that lets you use the earpiece as an additional speaker.
The earpiece isn't as loud as I'd like, but i suspect this is a software issue since it works fine with the audio mods collection.

It's microphones have good frequency response, and the noise cancellation is as good as advertised. 

Camera

OnePlus did a great job with the camera. The rear camera sports a 13 megapixel Sony Exmor IMX214 sensor with f/2.0 aperture, and uses 6 physical lenses to prevent aberrations and distortion. There are a handful of features that they added to the camera post-release through OTA. One of my favorites is the ability to capture images in raw format (files containing minimally processed data from the image sensor), as this allows incredible flexibility when it comes to editing and touching up photos. Here is a video by Tek Syndicate that goes deeper into why raw capture support is so useful :

And here is my amateur attempt at touching up an overexposed image :


This was pretty much a point and click, but because i had my finger on the lens just before the click, the photo ended up overexposed and washed out.


I was able to change the exposure considerably without causing artifacts by editing the raw file. Raw data leave more scope for both corrections and artistic manipulations, without resulting in images with visible flaws such as posterization.

The camera also has slow shutter, which lets you take much clearer images in low light conditions as long as you have either a tripod or very steady hands. 
Another feature they added through OTA is "Clear image" mode. Once you switch on the new Clear Image mode in the Camera app, 10 individual photos are stitched together for a final super high-res photo. Click here to read more about it on the OnePlus forums.

The One can record 4K DCI or 4K UHD (click here to know the difference) at 30fps, 1080p at 60fps or 720p and below at 120fps. The included camera app doesn't capture audio when recording in slow motion. There is a workaround for it, but it requires you to edit the video later.


The camera does heat up a bit when using burst mode, clear image or 4K DCI. I hope it doesn't cause trouble in the long run.

The front camera has a 5 megapixel sensor with an 80ยบ wide angle lens and supports upto 720p video recording

Battery and charger

It packs a 3100mAh battery that lasts suspiciously well. It can last close to two days with moderate use while on Dalvik, and it gets better with ART. (If you don't know what I'm on about, click here)
The charger is squircle shaped, which concerns me as apple can have it pulled off the shelves for infringing on their patent for rounded rectangles.
Jokes aside, the 2 amps charger is fast, and the cable quality is exceptional.

Innards

Powered by a 2.5 GHz quad core Krait 400 and adreno 330, you won't find any lag or stutter, and you'll be able to play all of the latest games effortlessly. 
The One packs 3 GB of 1866MHz RAM, which some would argue is a waste, as most of the time only about half of this is used :

I disagree with this because the remainder of available memory isn't left free, rather it is used to cache frequently used apps. This makes the system feel much more smooth and snappy. So the more you have, the smoother your device feels.

Software

Perhaps the biggest reason for me going out of my way to get the One back when  invites were as rare as sightings of the yeti was that it came with a custom version of CyanogenMod made just for it. I took that leap of faith and spent around 50% extra on securing an invite, and import costs because of the trust I once had for Cyanogen. The recent legal problems between OnePlus and Cyanogen are a cause of concern. I do believe that OnePlus is capable of making a good rom on their own, but it would have been better for OnePlus to hold CM to their contract and get 12S out of them while OnePlus worked on their own Lollipop build.

The One has a varied assortment of features, but unlike other OEMs, I find myself actually using these features regularly rather than as a gimmick to impress people. Allow me to list out some of the things i found most useful :

Screen-off gestures are perhaps what I like the most. Double tap to wake and double tap notification bar to sleep are a great feature, as they greatly reduce the burden on the physical power button. And now that I use Commandr (Read our review here) voice commands, I (almost) never have to touch the tactile buttons. There are gestures to open the camera, toggle flashlight, play/pause music and change tracks. 

Another neat little feature i like is the ability to adjust screen brightness by sliding your finger along the notification bar. This isn't unique to the One. It is there in the more recent builds of CM11 and it's derivatives.

Having a 2.4GHz CPU is great, but i dont find myself needing that much beef most of the time. The One lets you toggle between three modes, based on your power requirements, using a tile in the quick settings panel .

If the three presets aren't good enough for you, you can customize the maximum and minimum clock speeds, CPU governor and read/write scheduler to your liking
One comes with an app called AudioFX that lets you fine tune the sound to your liking. It has a 5-band EQ, reverb and knobs for bass boost and crossfeed ( labelled surround sound)

One of the most popular features of Cyanogen's ROMs is it's theme engine. CM11S comes with theme engine as well, allowing users to apply themes system-wide. There is an abundance of themes out there for CM's theme engine, and while most of them are paid, they are definitely worth it.

Theme : JB Extreme(Google Play)
Custom lock screen : Brushed metal with OnePlus logo by NunHugger (AndroidFileHost)

Conclusion

After OnePlus' rise to fame, lots of other manufacturers have released their own flagship spec'd low cost phones. Some have even gone with the 'no publicity is bad publicity' approach and resorted to lawsuits and controversy. But personally, I think I'll stick around on team OnePlus because the One wasn't about just making a flagship killer at a fraction of the price.It was a phone built by the community, for the community. They asked us for suggestions held long discussions with the community, and made the One to our demands. This is what sets OnePlus apart from other OEMs, and this is the reason i believe in them. 
The One is a damn good phone, in spite of how little it costs. It has earned the title of "The Flagship Killer - 2014"