Ahh I see....
It's odd that some of us joomla 1.5 guys are having problems with overlib. However, that said I am not sure that I like having too many js files all loading, the less the better. I am a bit biased with mootools and since mootools is being used (all over the place) sobi2 / Joomla / templates / etc. May as well go with mootools cause it's already being loaded for something in your CMS. Adding overlib is just another JS file adding to the bloat.
However, I know these guys have to have it there for backwards / forward compatibility. So for people like myself that can do light-work hacking it's nice to de-bloat the goat.
besides, the mootools styles can be a bit more fancy if your prefer that web 2.0 style.
Cheers.