Ive got a 40gb fat model from just after they removed backwards compatibility and ive had it for 2 1/2 years now. No problems so far but considering the way things dont seem to last the way they did years ago (i'm looking at you ipod) I wouldnt be shocked if it suddenly packed up. I use it quite alot too. If I can get 3 years out of it i'll be very happy considering the horror stories i've heard about earlier models of the 360, although I hear they're a hell of alot better now thankfully because I'm planning on getting one.
For a whole year I had the console inside a tight little cupboard with barely an inch on everyside of it. The fan would be on constantly for hours and hours and it was really loud. It took me ages to realise it was because the console was getting stupidly hot in such a small space and so ever since I move it out into the open on the floor when I use it and even if I play for hours the fan never needs to kick in. I reckon that mustve took some life out of it when it eventually goes.
