The question of whether it is "worth it" or not is impossible to answer with certainty since, ultimately, the decision hinges on many other factors such as the user's monitor, power supply, motherboard, what games they play and the settings/resolutions they play them at, budget, etc. IMO, all of this must be taken into account before making a ruling.
I used SLI 6800GTs for over 13 months. Ran perfectly. And it wasn't until the 7800GTX came out (8 months later) that there was actually a single card that could match the GTs (in some benches). So it was nice to be able to skip a complete generation and still have a competitive system for these modern games.
Now, as big a fan of SLI as I am, with my current upgrade, I went with an XTX instead of 2 cards. To me, this time, SLI wasn't "worth" it. The 7900GTs were nowhere to be found when I was looking, and, at $300 each, it would have been $160 more than the XTX I bought. The GTs only have 256mb ram, and I really wanted 512. And, as some benchmarks have shown, in Oblivion's outdoor areas, the SLI GTs pretty much run equal with the XTX... and that happens to be the only game I'll be playing for the next few months.
So, I was able to sell off my old 68GTs and get an XTX for $99. New 79GTs would have been another $160. And, in my mind, it just wasn't worth the extra money for me. Had it only been $50 more, I probably would've done it.