It's a great sub, no regrets at all.
I've also heard the 500 (at Axiom) and the 600 (at Mojos), both also great subs. I was concentrating on the speakers when at Axiom so didn't really notice any difference between the 500 and my memories of my 350 (which I had already had for about a year). I gather it has better linearity and should blend better (along with some other digital control benefits), but I have not noticed any such deficiency in the 350. The 350 will pound pretty good even in a somewhat large room (currently 30x14x8 opening to rest of house). It does not, however, really keep up to the 600 for all out pants-waving, chest pounding, gut-wrenching bass. Mind you it is about $1200 cheaper.
M80s/M60/QS8s/EP350; M22s; M3s.