Efficiency- and longevity-wise, pretty good.

They're fragile as heck, though, and contain mercury (albeit a small quantity in a relatively less-harmful form). Breakage needs to be handled appropriately, and disposal is as hazardous waste.

LEDs are more efficient, offer better (and often more flexible) light quality, are damndably rugged, and have far less toxic material load. Given the balance, I'd be swapping out CFLs (and have been).