The OnePlus One runs on a fast Adreno 330, 578MHz GPU, with 3 GB of RAM. It also features a 13 MP Sony camera, 4k video, a 5.5" 1080p LTPS IPS screen, good LTE support, and more. My model is the 64 GB one (Sandstone black).
The way you can get your hands in one of these puppies is via an invite. The companies behind these, are OnePlus and the newly founded Cyanogen start-up, but invites are scarce. It's possible that the company has probably placed a small order to be manufactured (which is normal for a first product by any start up), but the demand seems to have skyrocketed.
The size is definitely larger than your average 4.5" smartphone, which offers a better browsing experience. It's really impressive operating on-the-go at that size and resolution. At 5.5", you need a large hand to operate it with a single hand. Or, two small hands. People with too small hands like mine (I'm only 4'11") require two hands to operate a smartphone, so be that smartphone 4" or 5.5" size it matters little. Sticking it on my jeans' pocket and staying there without falling off, was a bit problematic though.
The other problem is that the native DPI for that size/resolution is 400 DPI, but Cyanogen has it set up as 480 DPI. There's a hack to change the DPI, but the Trebuchet launcher does not adapt the same way the Android native launcher does (so for example, you still have a row of 4 icons instead of 5). I hope this is fixed in the future, because it's literally wasted space. Right now, it's a 5.5" phone that operates as a 4.7" inch one.
Another problem of Trebuchet is that when you create more virtual screens, the Home button does not take you to the middle screen, but to the first in the list. This creates more unnecessary scrolling. The Nexus 5 operates the same way, but it does so for a reason: to access Google Now on the left most foremost screen. The OnePlus One doesn't have Google Now as a virtual screen (it appears only if you long-press Home), so it shouldn't copy that behavior.
Having said all that, I do love the phone for giving you the choice to have hardware buttons or in-screen buttons. I know many people prefer the on-screen ones, but I'm rather set with the "hardware" buttons, since I'm all about freeing up screen space as much as possible.
The call quality is great. There are three microphones, and a great stereo speaker too. I know that some people said that their call audio was too low in volume, but I didn't personally have any such problem.
The speed of the phone is phenomenal too. Installing apps (Google Play Store is included) and running them is a breeze. Gaming performance is very good too. On my previous phone, a Google Galaxy Nexus, browsing and installing was a huge drag.
The camera is among the strongest points of this phone. Quality is very good, colors are punchy. The 30 fps 4k video (and 60/120 fps support for 1080p/720p) is awesome too. Now, to be honest, there is an opportunity for the Cyanogen company here. I'm part of the indie enthusiast filmmaking community for many years now, and I know an opportunity when I see one. You see, for three hundred bucks, getting a 4k video camera is still a huge deal today. If only a few basic video features were added (should the DSP supports them), then Cyanogen has an opportunity to completely take over the niche of enthusiast indie filmmakers. All it needs is an exposure lock (no need for full manual control, unless the DSP supports it), focus lock, 23.976 (24p) & 25 fps support, flat colors that are important for post-processing (this means low sharpness/saturation/contrast, and a cine-gamma curve), and some more bitrate if possible. Bloggers like Philip Bloom, who are very influential in the indie filmmaking community, would be all over such features that no other phone offers! I have personally shot pro-looking music videos for indie bands with such limited features using super-cheap digicams, 3 years ago already -- but there's a minimum amount of features you need in order to achieve that. So far, no phone offers these "minimum requirements" for videographers. Some third party apps for iOS offer similar features for still pictures, but there's nothing like it for video in either platform. This is why I believe that Cyanogen can grab the market of a growing 4k niche here, should the DSP in the phone allows for all that.
Speaking about multimedia features, Bluetooth A2DP could be better. We're using A2DP to stream the high-quality audio from Spotify to our Onkyo receiver and large speakers, but the quality received is far worse than that of any iOS device (the difference is extremely audible). In fact, no Android phone today (and I've tested a few so far), can outdo iOS' A2DP quality. For the OnePlus One to not having MHL support either, is a problem for that use-case.
On the upside, there's a tremendous battery life. Sure, if you use the screen a lot, the battery will be depleted as expected (it's a 5.5" screen after all), but if you let the phone overnight, it keeps most of its battery intact. The reason I'm mentioning this is because on almost all Android phones I've used so far, I would wake up in the morning to find my battery missing by as much as 15%-25%, just by sitting there. The OnePlus One never loses more than 5% overnight.
Overall, I really like the OnePlus One. For me, it was the right purchase, given the various options around. It's a high-end phone with the right price tag. Continuing updates should be able to bring this phone to even higher planes. Let's hope that OnePlus will be able to ramp up the manufacturing speed.