Vocoders are just like synths... there are good ones, and bad ones. Just because Roland have one in the VP doesn't necessarily make it a good one. And the REAL fun with vocoders starts when you process external audio through them, rather than rely on their internal synths.

For some hip vocoder, check out Herbie Hancock on 'I thought it was you', etc...

Other good applications involve modulating pads and leads with drum machine or other rhythmical elements. The whole 'talking synth' thing is just the surface of what they can do.