The Nexus 5 is only $350. This phone is nominally cheaper, but I wouldn't say 'killer'. I will think about getting it next time I purchase a phone though (I use a Nexus 5 now).
OnePlus has 64GB of storage for $350, Nexus 5 only 16GB. It also seems to have a significantly better camera and much faster, too (Rear: f/2.0 13 MP Sony Exmor IMX 214 sensor, w/4K video recording. Dual-LED flash. 0.3s shutter speed) It also has a 3100 mAh battery vs 2300 mAh for Nexus 5.
The processor I don't care about since 801 is just a repackaged 800 anyway, like all of Qualcomm's new processors.
They're short shifting themselves here by not having a microSD slot. The biggest check against Nexus 5 I know of is no microSD, and that very small memory. 3G isn't as fast or as available as Google wants to believe.
Personally, my main reasons for not buying a Nexus 5 were the battery, the camera, and that I wanted a slightly bigger phone. The OnePlus One seems to take care of all three (although I'm still waiting for in-depth reviews).
edit: maybe not what most people are looking for, so I'll elaborate. To install google apps, you'll need to unlock the device and flash the gapps package. Some people don't want to go that route, so in that case there are two possibilities:
a) You can't have google apps.
b) If OnePlus gets Google CTS certified, then they can bundle the play store and other apps, signed, and bootloader locked. Oppo did so for the N1, so the N1 is a cyanogenmod phone with official google apps support.
I used to hate not having a MicroSD slot on my Galaxy Nexus. Then I realised that I don't really need to have that much music/videos/games (the only things that challenged my 16GB capacity) readily available on the phone. It is good enough for me that I can connect a USB storage device (thanks to USB-OTG) and access any large files from there -and that's something I actually find myself needing to do only in rare cases.
Having a reasonably priced 64GB version removes much of the need for an SD slot though, at least for most people. Plus given the direction Google is going, the external storage is becoming less and less useful.
That stuff is nice, but this phone is significantly newer (as far as mobile phone go). It looks like the Nexus 6 is coming out in November. I would expect it to have similar specs.
I have a Sony Xperia Z1, which has a 3000mAh battery, and while the battery life is pretty good, it doesn't really last a whole day. So I don't know about that 3100mAh with the 5.5" screen.
