You'll probably find yourself investing in others eventually. Not because the stock Sonar plugs are lacking - they're not, they're excellent - but because they don't cover every situation and flavor. It wouldn't be possible to cover all possibilities with stock plugins, because they'd have to bundle hundreds of them. For instance, the track & bus compressors that come with the ProChannel are perfectly adequate for everyday compression tasks, but you might find yourself investing in the PC2A Leveler as well, because it just sounds so much more musical for certain tasks. I do love the ProChannel EQ though, and haven't yet found a situation in which I found it lacking, EXCEPT the fact that for some reason you cannot have more than one instance of it in a ProChannel (why Cakewalk, why?).
If you're like most of us then you'll be addicted to plugs and having a huge palette of them at your disposal. I love trying new plugs and there are thousands of freebies out there. I've got a stack of them that I know I'm only going to use once in a blue moon (wow and flutter, anyone?), but I hang onto them for those blue moon situations anyway.
Some of the older Cakewalk/Sonitus plugs have a bit of a dated look about them, but they're still excellent. I can't think of one that I've thought was crap. I love the Softube saturation knob and the tube saturation too, I can't imagine not using them on a project.