Pros The intercooler definitely works to cool the 360, improving gameplay.
Cons It has a tendancy to wear out fairly quickly, especially if you play more than 6 hours per day on average. It also might induce faux red ring of death.
Summary I've been using intercoolers for a while now, and they are fairly effective. Note, I used plural. I've had to buy three of them now for my one system. Their lifespan is like this: 1-2 months perfect use. sounds quiet, cools the system, I can play oblivion much longer than before. 3-6 months it's starting to make some noise, but nothing too serious. 7-8 months it's now making a lot of noise, is much less effective, and occasionally when I load up the system I get the red ring of death. The issues of the third stage dissapeared completely when I removed the device, and later got a new one. All three have followed this pattern for me.
If you aren't a hardcore gamer, you don't need this anyway. If you are, seriously consider buying it, but remember that it's not going to be a permanent solution. One method I've heard used was combining the pedistal fan the Microsoft offers alongside the intercooler, but standing up the console is an excellent way to wear out the disc motor quickly. Your very best option is to keep your 360 on a wire rack or similar, and off the floor. That'll keep it well-ventilated and away from where dust will collect easily inside of it.