I recently acquired a Glock 21. It has its pros and cons.
1) 13 round magazine
2) Large grip and good angle
3) Long barrel
4) Glock trigger reset
5) Glock features: no sharp edges, big mag well, etc
1) Overall huge size
2) Grip is uncomfortable for many people
3) Wide, blocky slide is more difficult to conceal
4) Magazines are very large
The very long grip is difficult to conceal without lots of forward cant. It tends to print under just a t-shirt. However, a good holster can mitigate most of this.
I have noticed that the gun is uncomfortable while sitting in a car with bucket seats.
It shoots very well. The large grip absorbs recoil and I can get some very fast double taps. There is enough grip there to allow for a very solid hold, especially under recoil. I didn't like the grip in the beginning, but I now appreciate them.
If I were to change anything, it would be the connector. I would put a 3.5# connector to improve it. Ignore the lawyer's warnings; the pull weight will be around 4 pounds.