When one plus one was launched it was called as the flagship killer and it really was and I think it is still holding good now also.
Pros
1.The build quality is really nice its a little thick but the curved back helps gripping the very well.
2.It has a 5.5 inch full hd display and has great colour reproduction, its very sharp and protected by gorilla glass 3. It is very bright and visible outdoors in direct sunlight.
The bottom firing speaker is very loud for media consumption.
3.It uses snapdragon 801 with 3GB of ram and the performance is very good. It can run any app and all heavy games smoothly without any lag or frame drop.
4.It runs on cyanogenmod and is always upto date with latest android updates. Besides cyanogenmod provides a ton of customizations to make your device look unique.
5.It has a 13MP back and 5MP front camera, both perform very well. The rear camera can shoot 4K video. Both still images and video quality is above average.
6.It has a 3100 mah battery and it is very well optimized and battery life is very very good.
7.For the price it provides specs which are found on flagship devices of top brands.
Cons:
1.It does not have a micro sd card slot so you cant expand storage.