The Unarchiver isn't a new utility to the Mac world, and we reviewed an earlier version of it. It's the Swiss Army knife of unarchiving programs and can handle pretty much any file, no matter how ancient, you throw at it. The native unarchiver for OS X is pretty solid in and of itself, but it's nice to have a utility such as Unarchiver around when you need something to tackle anything from Amiga PowerPacker files to StuffIt Archive files.
There are a number of unarchiving options, including when you want to create folders for the extracted files or alter the modification date, and the ability to move the original archive file to the trash after expanding the contents. The one thing you can't do is archive stuff. If you want to do that, you can use OS X's built-in archiver, buy the venerable StuffIt Deluxe ($49.99) or spring for the US$19.99 BetterZip.
However, Unarchiver is a free download in the Mac App Store, so it's definitely worth having as one of your basic Mac utilities.