SLI/CF haven't seemed worth it to me since the days of the Voodoo 2.
You get slightly higher quality/performance in existing games at the cost of additional money, heat, noise, PSU issues and driver / game bugs.
Always seems better to put the money you'd spend on a second card under a matress, wait a year, then buy the best card available at the time (and if they're NVidia cards, relegate the old card to doing PhysX work if you want). Then you'll get new features in addition to better performance, with fewer driver/game issues. It usually takes that long for a game to come along that would actually need the power of two SLI'd cards, too.
Perhaps Crysis is an exception, but it always is.
If someone wants to spend lots of money building a machine to play that short (very good, but quite short) game over and over then more power to them. For anything else, SLI seems like madness to me. Heck, apart from Crysis my three-year-old 8800 Ultra still runs every game I've tried at 1920x1200 and high quality (though not absolute highest quality with the AA maxed, of course). (Crysis ran well enough at a lower res.)