Sort of the prototypical pie apple, and for a good reason. When baked, the apple (literally) explodes with pectin and causes the butter and sugar to gel thickly. When cooled, it would make the pie a more solid experience.
The flavor is a bit tart at as you might expect, but not so overpowering. It definitely is a strong supporting cast type of apple. You could (and people certainly do) make a pie of just this sort of apple, but there’s a pretty good chance that it would all turn into a bit of a mushy gel inside.
Given the tendency to gel up though, it is definitely the sort of apple that one should include in any sort of pie simply as a good binder for the other apple types. I would even suggest that chopping these up more coarsely and some of the more firmly textured apples more finely so that those would be suspended in the Granny Smith gel.