Generally, I see it like this. Norton provides a product that is supposedly supposed to get rid of all your computer problems: viruses, disk fragmentation, software uninstalls, registry cleanups, hard-drive bad sectors, and so on. These features are very valuable on Windows machines as Windows has ongoing, nagging problems with all of these things. Hence, Norton's is considered an "essential" product by many of its users in the Windows world.
On the Mac platform, the story is different. All of these things pose no real problems on a Mac - the design of the OS, with no registry, no uninstall process and no viruses, for instance - means that these sorts of problems that Norton is designed to deal with never really become problems.
The situation is made worse by the fact that on MacOSX, Nortons has been known to cause a wide variety of problems with disk integrity and compatibility, and as such is actually much more likely to cause problems than solve them.
Save your money and steer clear of Nortons. You're much better off spending the money on an effective backup strategy involving storing regular, comprehensive backups of your work in a safe location. After all, even Nortons can't help if your computer is stolen or destroyed, and anything else that goes wrong with it can usually be fixed with much less money down than what Nortons actually costs.