Hi
We did the Hop on Hop off prior to our Panama Canal Cruise in May. We paid for one day and got the second day for free which was great.
We did all three circuits as well as getting off at the beach and also in downtown. We used it as a mode of transportation too though it does stop running about 5-6pm so you can not use it to dine in another part of town.
There are two companies that run the HOHO - there is the Big red bus (a maroon coloured bus) and the original HOHO Bright Red bus. We did the HOHO Bright red business. They had headphones, free ponchos, water and great guides.
My two boys (8 and 11) LOVED the tour also which was great. Lots of laughter and the guides changed their storeis so that the kids were involved (particularly pointing out the celeb homes that the kids would know)
Both buses stop periodically at the Bayfront Marketplace which is a great place to stop for food (hard rock, bubba gumps, fast food, lots of other restaurants).
When we got off the bus, and had finished wandering around, waiting for the bus, we only waited for approx 15-20 maximum for the bus. (we are from a town where we wait for public transport for about an hour LOL)
hope that this helps

Deb