It seems that most steel frames have more than adequate fatigue life for most riders. So far, most aluminum frames are holding up well, too. If the fatigue life is as good as steel, that's good, but the failure mode of steel is preferable. Steel warns you for a long time before it fails.

That's not to say I recommend against aluminum frames. I own two. I don't ride terribly hard, and I have so many bikes that I don't ride any one bike very much, so I'm not a good frame lifespan tester.

The only experience I have with carbon fiber bikes is as a test rider.

And I have no experience with titanium frames.