In reply to Glyno:
I think a lot will depend on what you're using it for. For fine detail work, drawing and retouching (dodge and burning) they're far better than if you're doing things that require less accuracy.
They do take getting used to. Mine wasn't a Wacom and wasn't quite as accurate as I would have liked. But still better than a mouse for certain tasks. There is also the fact that they are pressure sensitive. You have to set this in the PS brushes palette to use it and if that's useful then that's something you can't do with a mouse.
A final possible advantage is less problem with repetitive strain since it's more natural, well as natural as drawing with a pencil and paper at least.
But if you don't require the accuracy a mouse might be all you need.