We are going to LV in mid-July, and plan to see Mystere. Should I reserve the tickets now? Or buy them when I get there? The seats I get from the reservation (Treasure Island website) are just OK. I wonder if I can get better seats there...

If you are getting "just ok" tickets now- I think that might mean the better ones are already sold.
By the time you get there, you will get worse or none at all. They do sell out in advance.
Where are you staying? Sometimes the hotel helps, we stay at the Bellagio, they make reservations for us. Good luck!

Ditto what Scarlett said. I went there a few years ago and I called ahead to see the exact date I could start calling for reservations if I was not staying in one of the Cirque show hotels. I think it was three months in advance for Mystere and one month for O. I called the first day and time I could for O and there were still none available. I did get good tickets for Mystere though by calling the first day I could for non-hotel guests. I hope you can get tickets because it is fantastic. I have seen three of the Cirque shows and the ones that are in permanent theaters are the best although they are all incredible.

Don't worry if you can't get better seats. The theater is small and there aren't any really bad seats.

You can purchase tickets for any Cirque show 90 days in advance via cirquedusoleil.com.

I wouldn't wait and try to get tickets as soon as I can.

Bob.