A group of 9 of us went on the Yellow Bus Tour whilst in Oporto on a cruise. It cost E9 per person (allegedly a 'group discount' but I think more likely just undercutting the Blue Bus guy who'd told us his was E10 pp!) and we certainly got value for money as we were on the bus for a good couple of hours and went both sides of the River Duoro. I notice a previous review said the buses were infrequent, and I agree, we didn't see many other Yellow Buses, but as we didn't get off it wasn't as much of a problem for us. Take your own headphones for comfort as the commentary is worth listening to (the incidental music gets a little repetitve though!)
The only complaint I have is that the seats, whilst appearing to be dry, obviously retain water, as we all found we had soggy bums when we got up! As it was a sunny day we dried off quickly enough, but it wasn't an ideal situation!
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.