I booked this through Expedia several months before I came to Texas, thinking it would be a good way to view some of the sights of San Antonio. The hop-on/hop-off tour itself was alright. It doesn't stop at too many places, but it's worth it if you can't be bothered to drive yourself out to Mission San Jose and Mission Concepción.
The stops are Alamo Plaza, Tower of the Americas, Mission San Jose, Mission Concepción, King William, El Mercado, Main Plaza. You get forty five minutes between pick ups - it might not sound like much time, but believe me it is plenty. None of these individual sites provide enough entertainment for more than that. I spent the morning travelling around and got off at three of the stops (both missions, plus El Mercado). The other stops didn't really appeal to me; King William and Main Plaza are just places in the city with pretty buildings, and Tower of the Americas is a visit planned for another day.
I did get the second day free ticket, but didn't use it. Even if you get off at every stop you'll have covered everything in less than a full day. The tour is simply not long enough or engaging enough to need forty eight hours for it. I also had a bit of an issue with the young lady at the kiosk in the Grand Trolley place in Alamo Plaza. On redeeming my Expedia voucher, she directed me to the stop by telling me it was "Over there, by the Menger hotel. You know where that is, right?" I did not. I had been in San Antonio less than eighteen hours. I had no idea where she meant. That wasn't particularly impressive customer service there.
Good points: gets you down to the missions easily.
Bad points: some of the stops are in fairly pointless places.
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