avast! Antivirus for Mac is a free version of the popular virus checking software that first made its name on windows. Note that this free version now replaces the paid version that was initially released for Mac.
Although virus issues are not as serious a concern on Macs as they are on Windows, the ease of use and efficiency of the Windows version bodes well. Avast! Antivirus provides pretty much most of the functionality you get in the Windows version but of course with the added bonus that it has a slicker OS X style interface.
However, be warned that as with many virus checkers, its not very clear how to uninstall it so make sure you have something like AppZapper to get rid of it if you choose not to keep it. avast! Antivrus for Mac is very easy to use and follow allowing you to scan in realtime or scan drives individually for problems and errors. Scanning is fast and efficient and you shouldn't experience any hang-ups.
However, you may notice that your Mac slows down a bit and is even unresponsive at times while Avast! is running. It does tend to consume CPU at time and there may be added processes that Avast! for Mac runs that cause this.
If you want to make sure potentially virus carrying mail attachments are scanned, then you must ensure this option is checked in system preferences. However, its only available for Apple Mail so there's no support for other big mail clients such as Thunderbird and Entourage. However, the biggest difference from the Windows version is that its not free for individual users. You can however buy licenses from one to three years.
Its debatable whether a Mac still really needs an anti-virus checker but if you are thinking of using one, avast! Antivirus does have a very good pedigree on Windows although unlike the latter, its not free.