I was shooting SSP with a G19, but just I made some modifications and now have to shoot ESP. I don't re-load so shooting anything in a .45 is cost prohibitive. We usually shoot 100 rounds or so per match.
I like the IDPA format because
1) It is "real world" scenarios. Although, I don't expect to ever have to fight off 8 threats in less than a minute.
2) It makes you think while you are shooting. This is probably the most important benefit. The best trigger guy may/may not win. You have to control your mind while shooting and plan your next moves under the artificial pressure of a time clock.
3) I can shoot it with my carry weapon. I'm not LEO or military, I'm just a guy who works in an office. I like being able to compete with the gun I carry with me every day. I don't have to spend a lot of money to a specialized gun or other equipment. Gun, mags and rounds and you are ready to go.