My VPAS10 is in the passenger footwell. I tried very hard to get the 12 to fit, but it ate more legroom than I was comfortable with (it's not a good sign when you don't fit into your own passenger seat).
It could be louder, the bass is mushier than I'd like, and it puts out basically nothing below 40hz. With that said I tend to be a very harsh critic when it comes to sound quality, as far as car audio goes it definitely punches well above its size category. Great for any music without synthesized bass lines - adds a good, chest-thumping* punch to bass drums and does a very good job of filling out bass guitar riffs.
It's certainly not the best thing for D&B, R&B, EDM, etc - but a far cry from having no subwoofer at all, especially if a full-sized sub in a proper enclosure just isn't an option.
* disclaimer - mine points more or less directly at the passenger's torso