The only major disadvtange I've noticed is that if you have a habit of lifting the mouse it messes up the tracking.
Some people say that if you move to quickly it messes up the tracking...i find it difficult to believe that they can move fast enough to confuse images that are 1/1500th of a second apart. Even if you could move the mouse a full foot (12 inchs) in 1/10th of a second (very unlikely) that's still 12.5 images/inch or a measly 0.08 inches between images. And that's with the older eye, the new ones have 1.3333 times as many images/second. I may be wrong, but in my experience the optical mouse has no fast movement problems. The problem comes if you lift it.
The major cause of lifting is hitting the edge of the mousepad, DON'T USE A DAMN MOUSE PAD THEN
Edit: Oh and a side note, I didn't really know I lifted the mouse until using the Optical, then I discovered that I do and it took me a day or two to stop, once I did it was fine.
I find that it tracks great as long as you can kick the lifting habit if you do it.
I stole my room mate's optical mouse for aobut a week whlie he was gone, I used it without a mouse pad and I loved it. I'm feeling the need to buy one myself.
The shape may bother you, but between MS and Logitech there are 5 shapes/sizes available, I'm sure you can find one you like.
That leaves slightly increased price as a disadvantage as well.
Advantages:
Better accuracy & Smoother movement
No cleaning, while this isn't a major issue, it's nice.
Cool blue glowing logitech emblem (on the one I wnat at least)
Overall I'm resisting the urge to replace my MouseMan Wheel with the Optical version, I don't really think it's worth it, but if it breaks my next mouse will be optical.