1. As noted above, they improve the weight distribution for better front-end control and traction.
2. They make you feel more "in" the bike instead of "on" it. I feel more in control that way for aggressive twisty riding.
3. They get you a bit more out of the wind at highway speeds (especially if you have the fairing).
4. if you have adjustable forks, you get much better access to the preload and rebound damping adjusters.
You can lose some effective steering leverage if you don't adopt the correct riding position (get your elbows down so when you push the force in more in the direction of bar movement and not "down" the forks).
It's a little harder to look around in traffic. Doesn't bother me though.
As you said, it looks cool so unless you find a reason NOT to do it.....
This comes up ALL THE TIME. I'd suggest searching on clio-ons for exhaustive threads.
Cycle Cats are generally considered the best (and most expensive). Works of billet art and very adjustable. They mount below the top tripple but have risers making their effective position the same as above tripple models.
"Redline" are cheapest and many people (like me) are happy with them. Get 'em at MotoWheels or SpareShack.
Other common manufacturers are Rizoma and Road Racing. They're both in the middle in terms of design and price.
I'd say if you have the coin, go with CycleCat. Otherwise, I'd tend to go the cheapest as I did; Redline.
I personally don't see much reason to land in the middle.