I played with it for 10 minutes when it first came out and since then have either disabled or hidden it. It just seems to do stuff that most people I know leave browser tabs open for instead nowadays. The calculator is rubbish (and Cmd+Space, c, enter isn't hard), Google does better work at conversions and weather, etc..
You don't even have to launch Calculator, you can just type mathematical expressions directly into Spotlight, and it will immediately display the result.
