I'm currently rocking on a used, purchased for $100 steel Gary Fisher Mamba hardtail, complete with commuter bike rack, kickstand, and pump. Looks kind of funny when I take it on the mountain climbs around my area, but it works great, as I'm not a big downhill rider, and avoid big drops or really nasty technical terrain.
This bike is 35+ lbs with all the stuff on it. Right now, I don't mind the extra weight, since I'm not racing at all on it, and using it for hard training, so it has been ok. However, I've recently gotten more into road cycling, and am enjoying covering larger distances faster, and started looking at lightweight mtn bikes, such as the BikesDirect FLY, which they claim weighs in at close to 20 lbs, or a 15 lb differential betweeen it and my steel bike.
Have any of you riders done a similar upgrade to a much lighter bike? If so, did it help a lot, a little, or did you not really notice at all? And is such a light framed bike risky on the downhills?