I did the tour in June, 2010. I didn't make a reservation, just walked up to the desk where you pay for it and got the next available time. They had plenty of time slots available even though the park seemed pretty busy that day.
I loved the tour and would do it again. It was very informative and I thought it was really cool to be able to walk around some parts of the ride and see things up close.
If you like the ride or are interested in the subject, you will enjoy it. I think it's worth the price. Like others said, if you are only going to be at Epcot for one day or you have a big party, make a reservation. If not, just go there in the morning you wish to take the tour.