I'll keep this review short and simple. I will not compare the canon 6d to crop sensor camera bodies, because well, there is no comparison. A better standard to measure it against is the canon 5d mark II, which many people are familiar with, and which were the first full frame camera bodies I owned.
Build quality: Yes, it matters, especially if you are a working professional. The 6d's have adequate build quality in my opinion, but are a bit light and cheap feeling compared to higher end models such as the canon 5d series bodies. The 5d's seem like they could get run over by a truck and still be fine, where as the fait of the 6d is questionable. If you treat your gear nice though, it's more than fine.
Ergonomics/button layout: I was disappointed here. First of all, canon does not give you the professional button layout as you get with the 5d and 1d series bodies, so if you are switching from a 5d II you will be a little annoyed with getting used to where things are. Second, there are things that are flat out missing, and these are to me, major flaws. THERE IS NO JOYSTICK. Yes, that's right, no joystick, so you have to awkwardly shift your thumb down to the touch pad to change AF points, and unlike using a joystick, you can't press the center of the touch pad to automatically switch to your center AF point. Changing AF points on these is a chore, and can cause you to miss shots in a fast paced environment. There is also no dedicated flash exposure compensation button, though you can set the 'set' button in the middle of the aperture wheel to this function- however, the FEC value does not display in the viewfinder, so you have to take the camera away from your face to make adjustments, which is annoying. The DOF preview button was moved from the front left of the camera to the front right of the camera though, which in my opinion is a good switch. I have this button set to change my camera between one shot and servo focus modes.
Autofocus: This is the biggest issue I have with these cameras. You get one cross type point (the center point), and the rest of the standard canon autofocus diamond arrangement has single orientation points. Some times, outdoors, those points work, but they aren't reliable especially at large apertures. If you are in a darker environment, forget it. I consider this camera to be a center point focusing camera only. To get good autofocus, you must shell out over 3k for a 5d mark iii.
Sensor: So why do I have two of these if it seems like all I've done so far is complain? Well, the sensor's image quality and high ISO peformance is just stunning. Aside from the canon 1dx, this is canon's cleanest sensor- even cleaner than the 5d mark iii. I can shoot these cameras all day at iso 6400 without worrying about it, often go up to 12,800, and when quality doesn't matter that much or I absolutely have to, I'll push iso 25,600. The dynamic range drastically begins to drop past iso 6400 though, but at 6400 and below it is excellent. You can definitely push the files taken at 6400 way more than you could with the 5d mark II.
Overall: The 6d is a great camera with incredible low light performance. There are some professional level features missing, such as good autofocus, a joystick, and an FEC button, though most people can work around these issues. If I had an unlimited money supply, I'd definitely go with the 5d mark III over the 6d because of those features (mostly, its incredible AF system), but I don't so I'm ok settling with the 6d's for now.
Most people aren't as picky, and would be 100% satisfied with the 6d.