It probably depends on who you ask. If you have ever had the Producer, Professional, or Platinum versions of Sonar you already have many choices of FX, in addition to Sonitus, to choose from the start.
Cakewalk bought Sonitus:fx from an up and coming company called Ultrafunk in 2003. They provide a good baseline effects bundle for all version of Sonar. But as Chuck said, good plugins never die!
They are good, but even Cakewalk offers some newer tools in the flagship Platinum now. But even so, the Sonitus fx still has a place for many. They don't have fancy GUI interfaces, but if you know what you are doing, you can get results.
Before you splurge on a FX suite, take a google around at the free FX bundles out there. There is some good stuff from Blue Cat Audio, Melda, and Tokyo Dawn, for example.