Sadly I haven't had any experience with the Nexus 6, but the Nexus 5 is possibly my favorite phone out there. It is built extremely well and has a really good profile. It feels like it should be in my hand. The screen size and dimensions were just right too. Even with my OnePlus One I might not mind swapping for the Nexus 5, even with the outdated specs, if that gets updated holy cow I would get one. My actual experience with a nexus device is the Nexus 10, a monstrosity of a tablet that was really nice to have but ungodly to carry. It was very nice though... being able to use it as basically a laptop was really nice, in games I could use it to get information on items or whatever, and I could read books or browse the web without feeling cramped. The stock android that came with and the upgrade to Lollipop were good and rather fun. Really liked stock android. My gripe with the OnePlus One is the camera, for some reason either the software or hardware in my phone or ROM just doesn't cut it properly and photos come out sub-par. I still have yet to find the reason for this. I should say that I don't find the OPO to be the best phone out there, it's damn good phone, but not the best. I think the Nexus 5 beats out the OnePlus One, and if I could get something like the iPhone 5C with android and none of Apple's idioicy, it would also be better. OPO just has slightly too large a screen and is slightly too thin for me... or I'm just too particular with the way a phone feels in my hand.