I know it's already been suggested, but I so recommend SimplyRAR over others. This app is rock solid and works well for my and my website because it splits up RAR's into split parts/etc/etc. I've noticed at MacUpdate that it's not been updated for a while, hope this utility doesn't die!! Give it a try. :-)
The only downside that I've noticed to using RAR (or maybe just SimplyRAR?) is it can sometimes struggle with folder names. Example, if there is a file/folder with a bracket ( ) in its name then something like UnRARx reports CRC errors. Odd... So i just remove these and keep filenames with regular characters and then I get zero problems.
If I'm honest, I wish I could use the Finder's ZIP to split up archives.. now that would be cool!! Stuffit Deluxe is too expensive and slow so SimplyRAR gets my vote!
You're right, UnRARx is a great tool for decompression!