Well Ken, I am using Cubase SX for the PC with great success, and recommend it highly. I just recently read a review in Sound on Sound magazine where the writer was talking about how smoothly the Mac version was running in his system. (OS X) He was pretty complimentary on how well the Mac version was running, and made some other comments about how other versions of Cubase did not work well on the Mac with the first release, but that they did a great job with SX for Mac.

Sorry that I have no first-had knowledge, but hopefully that is a little bit helpful.