I have a degree in mechanical engineering and I am amazed at the ignorance being spouted on here as if it were fact. Aluminum is a great material for mountain bike frames and is not likely to fail during the life time of most riders. That's why it is used for the vast majority of mountain bike frames. If you are afraid of aluminum, you should not ride a bike, given that the handlebars, cranks, rims, stem and other vital components are most likely made out of it. (Imagine what would happen if your stem or handlebars were to break suddenly).

There is a lot of confusion about fatigue limit. It is true that steel is better in this regard than aluminum, but it still doesn't make steel superior to aluminum in every application. Steel is not indestructible and many steel frames have broken in the past. So a picture of a broken aluminum frame proves nothing. I like steel frames (5 of my 6 bikes are steel) but I wouldn't hesitate to ride an aluminum frame if it were built by a real bike company (like Trek) and not bought from a department store.

PS - It is especially telling that the "The ARS", who is spouting so much nonsense, doesn't know that almost every airplane out there is made of aluminum. Please Mr ARS, go to an airport and find me just one steel airplane. Just one. I'm waiting...