Depends on the site content. For some kind of corporate/service type site like here or over at Infopop.com I would say no, don't do avatars. Many of the visitors are one or two time visitors that won't be back and won't care about avatars.
Now, if its some computer game/clan site where its the same group of people hanging out there, they probably would love to have avatars and I think it would make sense then.
To sum it up, if the site content lends itself to avatars then I would prefer to see them. If it doesn't its just a waste of time and disk space.