Personally, I don't know what all the hype is behind Fi subs. I think people like them because they play low and loud and are cheap. RE XXX is expensive, and the JL W7 is a little overhyped.
I still find the Image Dynamic IDMAX a tough sub to beat. With it's combination of price, SQ, and SPL, it takes the cake. It's a great sub, and a great one to start your audio system with.