Messed around with it on day one, struggled to get the dimensions right, and then they lowered the file size limit so even the ones I was working with couldn't be uploaded anymore. They're nifty but nightmarish to work with properly. Mine's just a display box of old avatars.
Profile covers are great, they're like a nice background tune to your avatar main dish. I understand why there aren't many posters using it, it's definitely not the thing that's most visible. But i think if you made the effort to have an avatar, you should go a bit further and get yourself a nice, colorful profile cover.
If you don't know what they are, hover over my avatar. You can change it in your profile page.