I've had my Pioneer shallow mount for close to 6 months now and have been really happy with it. So far, all of the people I've let hear it and have shown it to are always impressed. Plus, I just have a single cab pickup, so one 12" does a fine job at rattling it to pieces. It sounds like any other 12 I've heard, I mean, if you put a shallow 12 beside a regular 12, you wouldn't tell an audible difference. Bass is really nice, and goes deep. It also gets loud off of less power, which is an ideal option for people with lack of space (lack of space for sub probably = lack of space for amp.)
Just thought I'd throw in some experience from using one.