I've been perfectly happy with my intro-level steel-framed Gary Fisher Mamba so far. Sure, it's heavy as heck (30+ lbs) and doesn't have clipless or fancy XT/LX Shimano stuff, but it climbs like all heck and eats the downhills as well. I don't do any big drops or jumps, so that probably helps.
Obviously the more expensive bikes are lighter and I'd probably go faster, but I like the extra workout. I can't imagine needing more braking power than the brakes on this bike - they stop like a rock! Even the intro level derailleur does it's thing with aplomb on the climbs, especially the rear shifter, which has never acted up on me on a climb.
Just wondering if I'll REALLY appreciate a bike upgrade, to say, a bike over $1200? I don't need to go any faster on my trails (passing most of the folks around on their $2000 bikes on the uphills already!) and I'm not into superduper lightweight. (I usually carry a 5-7 lb water-filled Camelbak anyway - kind of destroys the point of nonracing ultralight!) I've become a pretty dedicated, regular mountain biker, and definitely don't consider myself a beginner anymore.
For anyone who's upgraded, let us know what you REALLY loved about your upgrade!