There's two sides to this aluminium coin: an Al frame (made of oversized tubes) might be a harsher ride, but it will also be stiffer, ie. transmit power better, esp. when yo're out of the saddle. The difference is more pronounced in large frames: long steel tubes bend all over the place, and there is a significant weight advantage to Al, too (up to 2 pounds!)
Plus it doesn't corrode much, though its lifespan is shorter than a steel's anyway.
At least that's the theory. Anyway, I will pick Al over steel any day.