Until recently, I had a late 80's Schwinn aluminum frame bike (now a thief has a 80's Schwinn aluminum frame bike, but that's another story). I had heard a number of horror stories about early aluminum frames--that the welds weren't properly done, or that the technology hadn't been perfected yet, that one should expect the frame to come apart any day, etc. I assume these kinds of rumors are being spread about carbon frames as well. Is there any hard evidence here? I know this is very hard to quantify because there were dozens of manufacturers, and there are/were lots of technologies being used. But is there any evidence, or is it just a cloud of Fear Uncertainty and Doubt?
I ride aluminum frames almost exclusively and have had no problem with them.
The welds are what tend to break first on most bike you hear these horror stories about, but this can happen with any sort of weld (though some forms of welding are more effective than others), so if a bike seems to be too good of a deal to have an aluminum frame, make sure to check out the welds, for those could very well be the deal breaker.
I know in BMX, aluminium frames are too soft. I had a frame and it was designed for racing but I used it for dirt. Did not take too long for the headset (was not setup loose) to ovalise due to it being not hard enough. I did bodge this with shimmys from coke cans for awhile. It's also not a wise idea to put pegs on an aluminium frame as it leads to bending the whole frame out of shape due to the force on the pegs.
I ride a 1999 Specialized Allez Pro with an M4 aluminum frame. It's a great frame - light and extremely stiff. Apparently it's had some problems with cracks around the dropouts, but I've had zero problems with mine. I weigh 160 lbs FWIW.
You can see others' opinions on this bike over at roadbikereview: http://www.roadbikereview.com/mfr/specialized/older-road-bike/PRD_18545_1610crx.aspx
Apparently it's a harsh ride, but it's been a long time since I rode my steel frame Miele with Columbus tubing in the 80's so I don't have a great basis for comparison.
A carbon frame will be in my future sometime, but I'm spending my $$$ on stuff that will actually make me faster in the short term. My Computrainer arrives next week :)