In order to use the Flexi City Pass we had to go to a tour operator in Mexico City named Julia Tours which we did. A woman at Julia Tours gave us a card called an iVenture card. We used the card to do the Hop-On Hop-Off part of the Flexi City pass and we enjoyed using the Hop-On Hop-Off buses. The woman at Julia Tours had told us that instead of taking guided tours we could simply present our iVenture card at museums, etc. and gain admission without paying an admission fee. The woman at Julia Tours had given us a brochure listing where we could use the iVenture card. However, when we tried to use the card at museums and other places listed on the brochure given to us by the woman at Julia Tours we had to pay the admission fee. In effect, the iVenture card was only good to use for the Hop-On Hop-Off bus. Therefore, of the 184 we paid for the Flexi City Pass I estimate that we only received 100 worth of value for the Hop-On Hop-Off activity and did not receive any value for 84 worth of the pass. Due to our experience I do not recommend purchasing the Flexi City Pass.
For a short visit such as mine, the logistics of this package are too much trouble. They expect you to pick up the iVenture card at the local offices, which is not convenient. Luckily, as I was arriving after hours on Saturday and wanted to book a tour for Sunday, they dropped it off at my hotel.
I chose the National Palace, Cathedral and Anthropology Museum as my first activity - this tour was great, our guide was fantastic so no complaints there.
However, I was not able to coordinate any additional attractions within the time I was visiting. While Viator gives you a long list of items you'll be able to choose from, what you don't see is that many of them only run one or two days. The evening bus is only Friday/Saturday, Turbos Cantinas only Wed/Thurs, Volcanoes only Tues/Saturday and so on.
The turbos was an awful experience - I rode it 4 times and each time had major customer service issues. Mainly, telling me they would go to X, then deciding not to go to X, and leaving me stranded after dark I would definitely recommend taking another bus as there are several hop on/hop off options.
I would have been better off to book the individual tours up front, rather than trying to squeeze the available selections into an already tight schedule. If you have a longer visit and plenty of time on arrival to sort it all out, then it might work out for you.