I had this sub for a while. Well, I guess I still have it on my floor!
It's got 2 6.5" subwoofers inside with an amp built in. It's durable, thin, and not heavy. It takes up very little space, you can pile stuff on top of it, and remove it easily.
The sound quality is good, but don't expect a lot of SPL. I tried using it along side 2 12" Kenwood subs, and I couldn't hear any difference whether it was on or off. Still, it makes a huge difference over having no sub at all. It makes for a really nice full sound. It's a bit overpriced, though. If the PO didn't leave it in the car, I would have never been inclined to buy it. That's just personal preference though.