I'd say yeah, it is worth the investment of buying third party plugins.
Don't go overboard though, people often get too caught up in the hype of gear and only end up concentrating on collecting as much as they can (I got caught up in the all too, dw). Really, all you need is one decent EQ, one decent Reverb, maybe a couple of different types of Compressors, and some sort of harmonic enhancer/distortion plugin.
EQ I think is one of the more important things. I love Fab Filters Pro Q2. Excellent interface, quick and simple to use, and a decent sound out of it as well.