We booked very last-minute and were able to get in for this tour. When we met at the train station, we saw 30-40 people waiting for the tour group. Luckily, the tour company must have a limit to the number of people they will have in a single tour group as they separated us into a group of ~15 for our tour. Our tour guide was a resident of Zurich and extremely knowledgeable. She welcomed questions, which she answered nicely, and spoke loudly enough for everyone to hear. We went on this tour after exploring Old Town on our own for a few days, and this tour took us to places that we wouldn't have found or even known about on our own. The only (minor) criticism I'd have is that the tour ended at least 25 minutes early, which made us feel that we didn't get our full money's worth. Our guide attributed this to the able-footedness and pace of our group, but surely there would have been other things or places "on back up" for us to have seen to fill up what amounted to ~20% of our paid time.

If you have time is is a nice way to see the city and get a quick run down of Zurich. Helps also get a feel of where certain sections of the city are and where you might want to come back to in your free. If you have problems walking this tour is not recommend as you have to be able to walk up hills and stairs. Not hard or strenuous but just need to be able to walk.

Either the guide needs to speak louder at busy locations (train station or the busier part of town) or have a radio as they do in Florence. Unless you were standing right next to the guide (impossible for the number of people on the tour) it was extremely hard to hear what she was saying.