Not necessary is rather a subjective view, more objectively it does address a lot of OS level security issues and from that perspective it's a valid upgrade to take.

Personally, if you are running mission critical systems at whatever level, I would wait until the release of SP1 so that the bugs are ironed out.

But my test machine running Vista has had no problems with hardware compatibility, has been stable, and I actually rather like the Aero interface it has to be said.