I just recently made the upgrade, and purchased Acid Pro. For anybody who is framiliar with Acid, It's pretty much the same layout, but the big changes lie deeper within.
Like 8 effects processors that use the direct-x system. (very cool indeed.)
Also the ability to save directly to MP3, instead of saving your work as a massive wav file, then ripping it to MP3, is another welcome feature.
One thing I haven't tried yet, is seeing if you are able to use MP3 loops, instead of wav-only.
Anyways, to make this short and sweet; Acid Pro is definately worth the price that it currently goes for.