I've got an Intuos3 A5 tablet and for some things its great and others it isnt.
For drawing and texture work in photoshop etc it is great.
For sculpting in Mudbox (and probably ZBrush) it is great
For anything else it is not so hot.
Navigation wise it is pretty decent but will never be as good as a mouse. It also takes a while to get fully comfortable using it to navigate with and this can put people off.

Simon