Well, it's nice and all, but it has some things to consider:
Power consumption, obviously.
Not all games play nice with it, so you can use just 1 card. New games often take a while to get proper support.
2 cards are noisier than 1, and produce more heat, putting more stress on your cooling solution.
You don't get 200% performance, more like 150% in good case scenario, while price still doubles (need bigger PSU, can save some money by not getting 2 high-end cards but mainstream).
Performance scales with resolution; the higher the resolution, the higher the performance gain. You might need a more expensive monitor to really benefit from it.
(to recap, basically pro: high resolution gaming with high quality-settings. Con: high entry price and maintenance, bit more likely to stumble into issues with specific games)
I'd say if you want to game at very high resolutions with all the bells and whistles, it's worth it. If you're OK with lower resolution, than a single high-end card might be better. If graphics are not a major factor, get a decent mainstream one.
Imho the biggest benefit is not the performance in the short term, but in the long term: I have a 8800GTS 512, still going strong. Something like SH5 maxed out with 8x AA and 1280x960 resolution (considered low to medium these days), acceptable frame-rates (30-ish). It's 2 years old, and I feel like it's time for an upgrade.
With SLI, you can put off an upgrade longer, because you have more graphical horsepower to play with. As time goes by, the graphics card is always the part that starts coming up short (given that you picked a decent CPU and sufficient RAM from the start).
Another thing you can do is make sure your system is SLI ready, get 1 card, and drop in another later on if you feel you need it. Benefit there is you might get the 2nd card for a lower price, since new tech has come on the scene in the mean time. Just don't wait to long; when a card is no longer produced, prices tend to rise again as the stock dwindles.
Decide for yourself what you want, just be aware of what it is and what it brings with it.
