I have been relying on Norton products for PCs since back when Peter Norton himself was shown on the front of each box. I find the products easy to use, easy to understand, and they have kept me safe from viruses since my first dial up connection circa 1995.
When I bought my first MacBook Pro about a month ago to experiment with Xcode and Objective-C (iPad and/or iPhone development will hopefully follow,) I wondered if I needed to install an antivirus package. But I find myself online with this machine much more than I expected, and I decided to purchase Norton Internet Security so that I wouldn't have to worry.
The installation was a breeze and within 30 minutes I had the product installed and fully updated. The UI is different than my PC based Norton products, but everything is easy to find and the software has been running without any issues. Better yet, my machine is running just as fast as it was before the installation.
My only knock on Norton products is that just after the installation you'll need to run LiveUpdate several times in a row to get all of the latest updates. The Mac installation did not tell me to do this (I am nearly positive the PC Norton 360 installation does) and I needed to run it 3 or 4 times to update every area of the product. But this is a minor quibble. I am very happy with the software.