Professionally - I love 'em. From a marketing standpoint they are fresh, eye-catching, memorable and one of those gotta-have-it new technologies.
Personally - they get annoying after awhile. Yahoo! over did it with the cars racing across the page....the ARS one here @ Oprano ROCKED until I saw it 4-5 times and it started causing the pages to load slow...
I think that Flash overlay is like anything out there - when used sparingly, it's a good thing. There's a time and a place for everything.
Let us know how they work for ya Naughty - I think it's better to try and decide it wasn't a good idea than to never try at all and think about what might have been...