I really can't answer the reliability question tbh. I know there are members on the forum that use both xfire and sli 24/7, who're sure to speak highly of both.
I'd only personally go for it if I had the bestest of the bestest single gpus out there and stumbled across another bag of cash someone had left laying around.
You got to think about extra power, room in cases, mobo compatability, drivers, windows, scaling (which is just as good for some games in xfire as it is in others for sli, although lovers of each will swear it aint), game support (which isn't always there), profiling - and then rely on the fact all these things work out AND go on to be reliable ... ... personally there's enough issues with single gpu cards to keep me going.
(that includes dual gpu cards btw, although I'd favor them over 2 cards even tho they don't scale as well on paper)