Fatigued-out? Steel has a fatigue limit - which if your stress is under that limit, it will NEVER break - no matter how many times the load is placed upon it.

Sadly, aluminum does NOT have a fatigue limit and will eventually fail no matter how small that load is.

Take a look at the wikipedia entry on Engineering Use of Aluminum.

Key points: Al is 2/3rd less stiff than steel. To combat this you can either increase tubing wall thickness or increase the diameter of the tubing. Increasing the tubing wall thickness increases the weight. And funny enough, with the same diameter tubing - you have to increase wall thickness by 3 - almost exactly nullifying the weight benefit of using aluminum. TANSTAAFL.

Instead, bicycle manufacturers increase wall diameter. That's why all aluminum bikes have Don-Walker-sized downtubes (ha!) with gigantic ST/TT/SS/CS to compliment. The downside to this is now your large diameter tubing is more suseptible to crippling/denting/cyclic failure.

So yeah, aluminum ain't all that. But uhhh....that is a good price on that bike.