I've recently bought netbarrier from Intego and I think it's a must because it's not only a classic firewall, it's a "bundle" of several tiny apps...
First, it's a two-way firewall that can blocks or allows incoming or outgoing connections and detects any spyware attempt to connect the internet from your computer.
You can also protect your data by encrypting any credit card numbers or passwords, define some rules when you are surfing the web, monitor the internet traffic by application or port, check the wifi hotspot status near you and cleans your browsers with caches, cookies, history, bookmark with washing machine
I'm very satisfied about this product and I recommend it if you're not happy with the OSX built-in firewall

Mac users have known for a while that there are few actual virus threats to OS X, unlike Windows users who would be foolish to connect to the Internet, plug in a USB key, etc. without security software loaded on. However, as more and more people convert, it's likely that more and more threats will appear for Mac users, and as many businesses switch over to using both, and indeed, people load Windows onto their MacBooks and iMacs, it's likely they will become concerned about, say, e-mailing a virus to a Windows user. Intego's VirusBarrier X5 steps in here, and provides the protection that you may want.

Installation was very simple, as with most Mac programs - simple double launch, a restart, and it appears in the dock and in your top right hand corner bar next to the date. Enter the serial code, fire up the program and enter your e-mail address and a year's worth of updates will be supplied. A first scan took around 15minutes to scan 250GB, which I didn't think was too bad at all, and the program has made no difference to the speed of the machine, something most Windows protectors can't say. A scary electronic voice informed me no viruses were found, which was also pretty cool - even though it was as I expected.

I haven't tested the program to see if it actually finds a virus, as I haven't downloaded a test virus, but I have confidence it would, especially after reading other reviews. So as a product, I think it's great... as for whether it's worthwhile and whether the price is justified, that is up to you. I have been a Mac user for over 4 years with no issues whatsoever of viruses, and would not have bought one, but it may help you sleep easier knowing it's there.