I've never used one before but what's the word on the rotozip tool?

I can see how it would be nice for drywall etc but I'm looking for a
better way to cut circles in 1/2" or 3/4" plywood or mdf board.
(rather than a router).

Are these working looking at? -- is there a circle jig that works with
it?

Certainly you're moving less material with it than a router which may
be a good thing.