Like most cathedrals, Tours' is well worth visiting. The stained glass ranges from the early mediaeval to the contemporary and the building houses one of the most highly regarded monuments of the early renaissance in France. On the day in July that we turned up to visit, we just missed what was apparently an impressive circus performance on the cathedral square involving a woman on a device that was a cross between a trapeze and a seesaw.
The first time we visited it was an absolutely arctic January day a few years ago. This visit was conducted in the far more pleasant ambient temperatures of early July, and was positively hot outside, cool and quiet inside. There was a steady stream of wedding parties entering and leaving. Some of the women were wearing the most extraordinary outfits, but unfortunately, one can't discreetly photograph or publish such things.
The cathedral building took 450 years to construct, and as such provides a nice romp through medieval and renaissance architectural styles, and especially stained glass styles, with the earliest window from 1267 and the latest from 2013. The recurring themes are of course Bible passages and the stories of the saints, especially Saint Martin and Saint Maurice, but you can also learn something of the society at the time the windows were made. There are royal symbols such as fleurs de lys, certain colours are significant, and there are scenes of everyday life. Stained glass techniques, although remaining basically the same over all this time, did develop a number of innovations which can be tracked using the windows in Tours cathedral. It's a remarkable and valuable repository of the history of stained glass.
I recommend taking a pair of binoculars if you want to get the best out of the stained glass. Most of the really old glass is very high and super detailed, so you need to look at it through the binoculars. There are numerous excellent panels explaining each of the windows.