The two circles Yellow and Purple were an excellent way to see Berlin. Although it could be done in one day, that would be almost too much. So a two or three day pass would be better in my opinion.
However, I have one significant complaint that was very frustrating. If we got off the bus at a certain stop, obviously we knew where to re-board. However, except for a very few numbered stops, there was no way to tell exactly where to wait for the bus if we walked around the city and tried to find the nearest stop. The map is essentially worthless. We wasted a good part of the second day trying to find a stop after we toured the Pergamon Museum and eventually took a taxi back to the hotel.
We used this as a great/easy way to get around the city and to the things we wanted to see. We had 2 different tours and made use of both of them. There was more narrative on the Purple tour - but less 'sights' to see.
Thoroughly recommend this option for multiple days
Difficult to find where to change your voucher for a real ticket if you're not trying to start the tour at either end, which are way out from the centre. The purple tour wasn't an open air bus so the upstairs was too hot to sit in in June. Apparently the roof isn't opened because of the overhead tram lines. We've done these tours elsewhere - New York for example where the stops are more easily identifiable so you know where to get on and there is a definite venue for swopping your voucher for an actual ticket. We finally found an agent at Potsdamer Platz but it wasn't easy