So there's only AN additional mission, but has new content (bikes, etc?) Could be worth it I spose.
I'll play GTA until the next one comes out, we'll all do it. New one comes out, we start on that. Not many people would go back to playing San Andreas or Vice City, etc after playing GTA IV.
And that's what makes this one even better than the originals, if they keep releasing content for it right through until a new release then that just keeps you even more occupied and entertained with GTA IV.
It's closer to 25 new missions not counting side missions (stealing/Selling bikes, helping people, gang wars etc), in a side story that intertwines with the main GTA IV story, new bikes/vehicles, new weapons, new online modes etc etc...
Look at it this way, your $100 game is still a $100 game, this $25 game/add on is just expanding the original and basically setting new standards in how far they can take it. Making an already great game better - I'm all for it.
I reckon it's worth it.