Lifehacker did a pretty thorough write up of wireless syncing. In short - WiFi takes longer, but it's not too bad.
EDIT: Giant caveat - they only tested this over 802.11g. I imagine USB might still be faster, just because even though the iPhone storage is flash, we don't really know what its actual write speeds are.
While music and movies take 13 times longer over Wi-Fi than over USB, it's still a relatively short amount of time—6 and a half minutes for 4 albums isn't bad. If you aren't rushing out the door, it's certainly worth the "not getting out of your chair" factor, and if you're only syncing one or two albums, it'll be even faster. This benefit grows with something like the iPad, which is more difficult to just keep docked at your computer all the time—instead of finding it and bringing it to your computer all the time, you can just hit the sync button on either device and let it go to town.
Overall, Wi-Fi syncing is mostly what we expected—a slower version of USB syncing. However, if you're syncing a small amount of media, it doesn't make a huge difference, and if you're syncing apps, there's almost no reason to plug in at all. Also keep in mind that we were using a wireless G network, and if you're on the newer wireless N standard, you might notice even faster Wi-Fi times. And with the ability to keep using your device while you sync, it is very freeing to have Wi-Fi sync around, as long as you don't have anywhere to be. Have you tried Wi-Fi sync on iOS 5 yet? Share your thoughts with us in the comments.