My mother rides two custom magnesium frames, both by Segal: One road, one tri bike. Both weigh pretty much the same as a carbon bike - after her Madone was stolen, she had a choice for a fresh start and went with the Mg frames. Several of her friends are now switching, including her trainer.

She competes regularly in triathlons at the top of her age-group and commutes 70km a day by bike and the bikes have been rock-solid and reliable. Sure, the welds don't look as elegant as lugged steel (they're uglier than my Trek 1.2's TIG welds), but if it works, it works.