I just received the Google silicon case (in "gray," might as well be black). It's a very good case. I like the feel of the silicon -- smooth but not slippery, and grippy. My only complaint is the relative lack of protection for the bottom of the phone. The corners are protected, but the rest of the bottom is exposed. If it falls flush with whatever surface it's striking, the corners will protect it, but if it hits the edge of something (e.g., a desk, etc.), it could suffer damage.
Still, I don't regret getting it, and it's a vastly better OEM option that the crappy clear plastic case. I returned that and got the Spigen Ultra Hybrid Crystal Clear case, which I recommend without hesitation if you're looking for a clear case.
Edit: one thing I've noticed with the Google silicon case -- the silicon covers what is likely a hard, thin plastic shell, which is lined on the inside with a thin, almost microfiber like material. There is no plastic shell underneath the volume button area, though, leaving just the silicon and lining in this part. It flips away from the phone quite easily here. I first thought I had a defective case, but the fact that it is directly on top of the volume buttons cannot be coincidence. The power button is covered up normally, however (and works fine), so I'm wondering what the point of the non-stiff area on top of the volume button is. .