I think the custom avatars are in general a great idea.
HOWEVER...unless the site has the appropriate upgrades for bandwidth etc, it will result in slower downloads and connections of pages.
I would definitaly not want them if that would be a result.
The other problem is once you offer avatars, everyone will be asking for personal banners on the bottom of their posts. This can double or triple the problem if it is allowed. Not to mention the fact that most members really don't have any interest in reading other peoples personal advertisements on the bottom of every post.
I vote for keeping it simple...this isn't political forum, or a fashion show, I don't see any need for the "flash" if it will distract in any way from the performance of this website....Just my opinion.