Personally I don't know what all this 'SLI is problematic' thing is all about. I've SLI two GTX580s back in Feb and have had ZERO problems (and i've never SLI or Xfire anything in my life, I was a complete noob just wanting to see what the fuss is all about).
Fitting and installing them was as easy as fitting and install anything else you have in your case.
Both cards were picked up fine first time, NVIDIA driver found both cards first time, no error messages, no HEAT problems at all while playing BF3 on full Ultra preset (bout 70c top card 62c bottom, 55% fan speed, V-Sync on), even overclocked both of them no sweat, and EVERY game I have thrown at it have worked first time (and they run QUICK)
I can't comment on xfire as i've never done it, bit I say SLI FTW, if you want to go SLI then I highly recommend it (just make sure you have a case with good air flow)