I don't know if it's the same all the time, but when I went a few weeks ago (and it was a really quite time of year) the service was terrible!
We booked for 8.30 and arrived early, but, when we got there, the bus was already full. The driver had let people on who'd booked for other times (every 15 minutes between 8.30 and 10).
Anyway, he made everyone who wasn't supposed to be on, get off (he hadn't checked reservation numbers or even names). Still, a lot of people who had booked didn't get on and had to wait. The rest of us sat on the bus for 30 minutes in the parking lot!
My db and were the last to get on and had to sit at opposite ends of the bus.
We were really annoyed, as we payed a lot for our Flexitickets and were promised a standard of service which was not delivered.
The bus back was an hour early on this occasion, so after spending all that money on tickets and having a pretty bad trip there, we arrived 30 minutes late and left an hour early (although after all of this, we just wanted to go back to the hotel anyway).
From what I could gather, the bus company on that day was not the usual company, which was why we had all the problems, so hopefully you won't have the same, I just wanted to warn you so you'd be prepared if it happened to you. That way, at least it won't spoil your day like it did ours.