viagra - most data, most used, best side effect profile; DO NOT USE WITH NITRATES... bad!
levitra - wannabe viagra, didn't have all that much difference from placebo (and the clinical trials excluded anyone who'd ever tried and/or failed viagra... sounds fishy to me).
Cialis - been used in europe for a while, is being marketed as "the weekender" as it's half life is 36hrs... however, one of the CI's listed is alcohol INXS (i think specifically 5 drinks or so)... so if you're planning to go away with the missus/mistress/mister/sheep/whatever, don't plan on the bottle of wine with dinner...
also, none of these change the refractory period - so even if you have the potential to get a woody, it (a) still needs stimulation and (b) won't reload any faster....
might also add that one needs to r/o neurogenic/psychologic causes first
hope this helps,
-tim