If you're not familiar with a place, these guided bus tours are a nice way to get some history of the area. It's also a great way to get to and from a lot of the sights since you are allowed to get on and off the bus for the whole day. During the time we were there (March 1997), tours ran about every 15 minutes, starting about 9 am and continuing until about 7 pm. Unfortunately, we were running out of time, so we just rode it for the complete circle. That took about an hour.
Highlights of the tour were Edinburgh Castle, the Palace of Holyroodhouse, Princess street, and the Royal Mile.
Guide Friday also had a promotion through which you could get a discount on another tour in another city, or another tour in Edinburgh by showing your Guide Friday ticket stub. So we took advantage of that in Bath. I believe the savings was £1/ticket.