+1 for MacPAR Deluxe. Great little app to handle RARs and PAR2s. It's just been updated to allow setting a different location for expanding RAR archives. Great for those huge MKVs
I like BetterZip. That said, it's a full-featured archiver/unarchiver that supports multiple formats and costs $20. If all you want to do is extract RAR files, then I would agree with the many here that suggest UnRarX.