My favorite belay device so far.

I'd give it a 4.5 if I could, but since I've never used anything worthy of a 5, I'm grading on a curve.

I find it works as advertised. The main caveat is that you have to use a round stock carabiner, like a Petzl Atache. This has been said before, but people still miss it. Any time you hear complaints about it binding or not working smoothly, I promise you it's because someone ignored the directions and used some funky shaped forged biner with. So go back and read that part again.

As a regular belay/rap device, the b52 is smooth and amazingly consistent from one rope size to another. I'm fairly heavy, so rapelling with devices that don't give enough friction is scary. The B52 is better in this regard than both the standard ATC and the Reverso. What's great is that it gives a workable amount of friction even with skinny half ropes. My Trango Jaws is even better for this, but the b52 is plenty good.

For autoblocking it works as well as anything else. It has the advantage of working well with a wide range of rope sizes. It has the same disadvantage as all the others: the procedure for lowering a climber. Unfortunately, it will take a radically new concept to fix this problem.

The only annoyance I've had with it is trying to use it with a stiff, lumpy, fat gym rope. It bound up badly, and I got better results with an ATC. This shouldn't matter to too many people anymore, since most gyms now require a gri gri.

This is the uncontested Mack Daddy of belay biners.
However, after a while I realized I just don't need the Mack Daddy of belay biners. A smaller one just seemed to make more sense--lighter, less bulky, easier to manage. So now I use the Atache, also by Petzl. All the good stuff from the William, in a smaller package. If you need the big size, William is unbeatable. Otherwise, I strongly recommmend the atache. By the way, William who??

The best I've ever used. Smaller than most, but big enough to clip through a bunch of slings or use with a munter. The key lock is indespensible. There's no going back to regular hook and pin biners. The round stock is also important. With a lot of belay devices (especially autoblocking ones) round stock biners like this give you the smoothest feed. The red warning strip is nice to have. And the build quality is first rate. What's not to like?