I haven't seen any issues with simple mode, but I would echo that. It can seriously mess things up if anything goes slightly wrong, so you need rigorous testing

I haven't seen any issues with simple mode, but I would echo that. It can seriously mess things up if anything goes slightly wrong, so you need rigorous testing in place. Advanced mode can remove code you're using, it can rewrite variables you don't want rewritten, etc so you need to be sure everything is understood before you start. But the rewards are worth it... No compression: 92K, YUI: 56K, Closure Advanced: 47K.