Never tried unRARing stuff with Stuffit, but I don't think I'd recommend it, either. There are two great shareware programs designed specifically for RAR archives, and both work great and handle passwords just fine:
1) UnRARX (
http://www.versiontracker.com/dyn/moreinfo/macosx/15272 ); free, lightweight, works great. A new version was recently released, which I haven't tried yet, but it adds support for PAR2 as well (already handled PAR1). At least on the older version, you had to input the password before you began unRARing, though--be careful you don't miss that step.
2) MacPAR deLuxe (
http://www.versiontracker.com/dyn/moreinfo/macosx/14338 ); shareware, but does just about anything with RAR and PAR files. Nice GUI, too. If you use it on PAR2 files, make sure you don't run it on a group of files in a directory with a lot of unrelated files--it goes through and checks EVERYTHING in the folder to see if something is a miss-named RAR or PAR part, and this can take a very long time on a directory loaded with large files (I kept thinking it was freezing till I realized what was going on).
Gumby is an all-in-one tool that can also do some of this stuff, but I haven't had much luck with it yet.
