Depends.
If you're a serious compositor, or want to get into heavy compositing, Boris is the route to go. Add to that 3D containers/shapes, etc, it's REALLY the way to go. Vegas comes with a light version of Graffiti; Boris Red is really just Graffiti and FX combined.
If' you're not into deep composites or don't plan to go there, then you probably won't care about FX.
Thanks Guys. I imagine that the compositing features of Vegas are more than enough for me. I'm more interested in filters / effects so it is probably not worth my while.