I used to be a 9mm fan because it was cheap. Then I was a .45ACP fan because it was mellow to shoot. Now iu like the .40. Not that I dislike 9 and 45, but my shooting has developed, and .40 is the best fit.
9mm recoil isn't bad, but it has a lot fo muzzle flip. It cyclles the slide fast though, which is good for quick follow up shots. It also shoots pretty flat making 30-50 yard shots simpler (i.e. for USPSA, you aim at the a-zone like usual and don't have to think much about it). If you reload, the cartride is a PITA. The case taper, bullet profile, and general small size of the components makes it annoying to reload, not to mention you can get military crimped brass mixed in to your pile of brass. On the upside components are cheaper.
.45 is easy to reload. Low pressure and long lifetime of components make it simple. The only real downsides are cost of components, military crimps, and having to watch out for double charges. BUt mostly you can work up a full zorch load or a light without much worry and load it fast. The recoil is mellow. However, it torques more, it takes a while to cycle the slide, and it is more work to put it on target in the 30-50yd range.
For my uses the .40 is a good compromize. Bullet weight can vary from being near the heavy end of 9mm to the mid-range of .45. The cartridge has much less taper, and the components are a bit larger, so laoding is easy. You won't find surprise military crimps in your mixed headstamp brass. Loaded to minor power factor it is VERY mellow. Even loaded light, it cycles the slide qucikly. Even with heavy bullets it is less torquey than a 230gr 45. Even with heavy bullets, it tends to shoot flatter than .45 making 30-50 yd targets a bit simpler.
It's agood fit for my shooting, and given the dimensions, I'd probabl carry it in a self-defense pistol if I lived in a CCW state just for consistency with what I'm used to shooting.