General consensus does seem to be that Crossfire/SLI are to be avoided unless you've already reached a performance threshold with a single card. In other words, a single more powerful card is better than two less powerful cards.

That being said, I've gone the Crossfire route with the 4870, 5870, and 7950 series and I don't regret it. But, there are four main issues that you'll be likely to run in to under Crossfire/SLI that you won't have on a single card:

• Power. Running multiple cards means you'll need a beefy power supply.
• Heat. This is a big one. Unless you've got an extended ATX motherboard, chances are that your cards will be in adjacent PCI-E slots and there will be very little room for airflow between them. That means the temperature on the cards will be much higher and your ambient case temperature will rise as a result. You need good cable management and good case airflow to make sure you don't overheat.
• Microstutter. Especially with AMD cards in Crossfire, you're going to get microstutter. That's basically when your fps are high, but frames are getting rendered at inconsistent speeds. Maybe you've got 50 fps but one frame renders in 20 milliseconds and the next one takes 100 milliseconds. It wont decrease your fps, but it will result in a minute stutter that can be quite annoying in some games.
• Crossfire Application Profiles. In order to take advantage of Crossfire performance, AMD has to write profiles for each new game that gets released. They're generally pretty quick to do so, but if you tend to purchase games on release, sometimes you'll have to wait for a profile to be released to get the best performance.

Power and heat problems can be solved. Waiting on application profiles for new games isn't generally a problem. But, microstutter is not something you can fix or get around currently. Thankfully, AMD is aware of and working on the microstutter issue. The 13.8 beta drivers that they released on August 1st have a new feature called Frame Pacing that helps cut down significantly on microstutter, but it currently only works at resolutions up to 2560x1600. The next beta is supposed to include Frame Pacing for Eyefinity resolutions, but until we see it in the wild we don't really know how well it will work.