Sometimes, just to show off for friends. It'd be more useful if it didn't seem to slow down my iMac (2GHz G5 w/ 2GB RAM) and close itself all the time. Plus, even with the key word enabled, sometimes it responds to my TV and very loudly announces the time or moves to the bottom of the page. Or opens some random app.
I hear it's better in Leopard, but I didn't get a chance to play with it when I last used it.