I haven't used MESS before but I'm looking into it pretty seriously. I had two questions though that I wanted to clear up:
1) I was wondering though what the 'status' means as shown on http://www.mess.org/supported_systems? Does 'preliminary' mean zero-few games are playable, then 'imperfect' is some games are playable, and 'good' means most are playable? That's what I would assume...
It varies.. SegaCD is still marked as preliminary, but I'd say 80-90% of the titles work fine. (I don't like to mark things as working until I feel they're on par with other emulators)
Jaguar was at one point marked as working, and I think 2 games in the entire library run!
Try it and find out...
2) My second question has to do with layouts for artwork. My understanding is they are nearly identical (in function) to how MAME does it. Is this correct?
MESS is identical in function to MAME, they are exactly the same piece of software with different support lists. MESS presents things slightly differently with the systems / software list concept but this functionality exists and is used in MAME too, for example the megatech multi-cart support.