For pure fun, the Mac10 is hard to beat. One of the fastest cyclic rates in town. I went to a rental range in Nevada, and shot a bunch of auto's, and i think the Thompson (Tommy gun) in .45 is the most fun, and secondly would be the Uzzi, and then the Mac11 (smaller 9mm version of mac 10). Way fun to shoot. Id personally rank it about an M16 in fun factor.
$900 sounds like a great price from what ive herd. Most good condition M11's are going for 1500 or so now, and have steadily been going up in price lately. Its the current class three bargin out there, andthere are many parts for it, but mags are somewhat costly...but there are lots of conversion type thing for it too shood greese gun mags,- its a good buy id say. Get it, and love it.