My biggest grievence with multi GPU's is the inevitable micro-stuttering.
You can argue that the performance increase is worth it as much as you want, but the moment you are forced to play games with below 60 FPS you will run into microstutter.
This leads 30 FPS on a single GPU to be far smoother and more playable than an SLI/xfire setup with 59 or less fps. And yes, I do say this from experience (SLI 285M's, SLI 460M's).
My personal opinion: avoid multi GPU's at all cost.