I've never been on one, but on other forum boards I've heard it discussed. The general impression I get is that it's about like you'd expect: you have things arranged and you can see more things because of it, but you lose out on flexibility because you travel in a group. Also, the other people on the tour and the leaders can make or break the tour.

Most people seem to advise taking a Contiki tour if you've got very little time and want to see a lot of things, if you're nervous about traveling by yourself, or if you want to get an overview of an area before traveling around by yourself.

Stray Travel is another group that gets mentioned from time to time on the Eurotrip board, but I haven't looked into them. It sounds like a hop-on, hop-off service, but maybe with a little more organization on top of it. Maybe. Or it might be like Busabout, I'm not sure.