Audacity for Mac is a powerful audio editing tool that surpasses default OS X tools. The downsides are a dated and sometimes clunky interface and a cumbersome installation process.
Powerful and robust feature set: Audacity is loaded with audio editing, recording, and exporting tools that work in tandem with Mac's multimedia capabilities.
Free with regular updates for diverse uses: Audacity is a free, regularly updated tool with solid support from the developer. This means a solid core that works far better than some other free software suites in the same category, and a good alternative to somewhat limited tools like GarageBand.
Interface is dated and sometimes clunky: While Audacity's feature set delivers, it looks and feels a little dated compared to more modern software. In most cases this is purely aesthetic, but finding certain menus can be cumbersome in some circumstances.
If you need a powerful, free audio editing tool for your Mac, Audacity is a great tool with which to start. While the interface could use some updates, you'll still get the bang you're looking for in your audio project.