You get what you pay for ...
We bought the $99/person Hoover Dam/Lake Mead tour. All in all, it wasn't a bad trip. We spend 10 hours on the bus total, but our driver made it bearable by explaining the area around us while were driving to our destinations in the morning (as well as some fun Vegas facts -- we drove by Wayne Newton's House!).
Bad part ... We only got a very short tour at Hoover Dam -- no time to look around because we had to be back at the bus right after the tour. Some folks on our tour were very upset by this and harrassed the bus driver. The Lake Mead tour was nice, but the food sucked. If anyone wants to take a tour that includes food, I suggest bringing something with you or buying food at the many stops along the way. While our driver was great at the beginning of the tour, his mood turned sour after dealing with snotty patrons (rightfully so). We ended up driving around Las Vegas for 2 hours after our tour while the bus let people off at their stops.
In the end, it wasn't bad, but it wasn't great either. Regardless, we got to see beautiful Lake Mead and the immense Hoover Dam!