I haven't actually used it, but AIM 5.0 sounds like the tackiest thing ever. They're making "themes" (i.e. really dumb-looking skins) for it, and I hear they're trying to cram advertisements into the actual IM window. Blllarrrgh. I know there are ways to get rid of the advertisements, but... feh. There aren't any new features in the new version that seem significant enough to bother myself with.
Besides, 5.0 isn't yet compatible with
AIM+ yet. [Side note: If you don't currently use AIM+, I would advise you to click that link and go download it right now. It is godlike.
]