I really wasn't impressed. The GPS on my phone said the bus was going at 45-49mph, which it could have easily have managed on the A14, and given that disruptions on the busway are more likely to be serious than disruptions on the A14 (harder to drive around), even if less frequent, I'd rather take my chances on the A14.
I didn't think the ride quality was that impressive - I'm pretty sure I felt every joint, and the people around me were moaning about it too.
I was on the first bus out Cambridge, which wasn't that busy and certainly wasn't close to full, but it also lost extra time on top of it's already late departure time while on the Busway.
The return journey from Huntingdon was very busy and full most of the way from St Ives. It lost 27 mins over the course of the trip, the majority of this being on the guided section!
There seems to be a lot of small teething problems to be ironed out. The traffic lights in Orchard Park stopped the bus at every junction, one set of traffic lights on the guideway was being controlled by someone with a laptop on top of the control box, a lot of the ticket machines weren't working, many of the electronic displays were giving wrong timetable information (ie at Drummer Street) or information for the wrong time of day, and some of the publicity material is confusing (like the routemap being upside down and Cambridge station appearing on it twice).
There's also some usability issues with the ticket machines, like the destinations likely to be most commonly travelled to being hidden well down the list of options, and them only selling four tickets at at time (which could be a problem if you wanted to buy five tickets by debit card and the fifth one was under the £3 transaction limit).
Lasting impression would be that I can't believe they spend so much on something that's... not that wonderful.
I live hoping that one day it will see tram trains that will finish up with an on street section from Cambridge station to a loop round the outside of Parker's Piece. Other countries can do it; we ought to be able to do it too.