I'm sorry to say that NOTHING like MJ exists for the Mac. I've been a die-hard Macintosh user for nearly 13 years now and I'm ashamed to say that I built a PC to integrate into my home theater for running MJ.
iTunes does work well, but it doesn't nearly have the extensive feature set that MJ does. The only other MP3 player I've run across that comes close is Audion from Panic Software. Even it falls far short of MJ.
As for other Macintosh options...it does well as an MJ remote by running VNC client software or by using the web remote/Glissando combination. Glissando runs a little slower on the Mac, but that's because Flash runs slower on the Mac.
I did get MJ to install using VPC, but I didn't try running it with local media files. I know that the VPC/MJ combination did not work well with networked storage of media files--it had to buffer every 3-5 seconds on an 802.11b connection.
What has worked for me was to use MJ as my primary axe & use the conversion feature to maintain a seperate directory structure of MP3 files for Mac/Portable use. (My main files are all APE). I can then point iTunes at the alternate directory structure, while my main files contain all the extra MJ info. Backing up the iTunes database gives me peace of mind about any tagging/rating that I've done with iTunes.
There are lots of tag editors for the Mac. Check out VersionTracker for a comprehensive list.
Best,
Brad