I'm with Banzai--you can get good or bad frames in any material, though the incidence of bad in Al may be a little higher than others because so many low-end makers use it these days.
I have several steel bikes (going back to my college commuter, which I've never bothered to get rid of) and only one aluminum, a decent Cannondale mountain bike. It's hard to make a direct comparison because the steelies are road bikes with higher-pressure tires, while the Cdale has knobbies at 45psi, but they're all fine for what they do.
My only real concern hasn't ever come up: I used to ride a lot harder than I do now, and also carry the bikes loose in the back of a pickup (I was poor and aggressive then; I'm richer but older and slower now). I used to worry that the alum tubes would get dented or a dropout or something would get bent. Steel can often be bent back or repaired, while aluminum can't. But it never actually happened, so who knows?