I was a longtime .45 shooter who only finally grudgingly accepted 9mm ... when they made me carry one for work.
Just about 10 years after the .40 had been in LE-service, and had demonstrated it was "working" acceptably well in actual shooting incidents, I became a little interested in it. That "interest" was motivated by the fact that my own agency was beginning to show some interest in it. I'd already picked up 3 different makes/models before I was issued my first one.
Now I own 5 of them, which is coincidentally the same number as the 9's I presently own. (I own 9 .45's, FWIW.

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It can be a somewhat snappy and brisk recoiling caliber for the "average" shooter, and a bit daunting and unpleasant for the "minimally average" shooter who isn't interested in shooting very often (or more than is absolutely required, if it's issued).
For an armorer required to maintain and repair a lot of issued weapons, it's often a bit more attention and parts-intensive than maintaining a similar inventory of 9's or .45's.
For firearms instructors it can make it a harder task in training and repeatedly qualifying folks who aren't more than "average" shooters (especially if the shooters with whom they work aren't "gun enthusiasts" and their interest stops when they manage to squeak by qualifying and leave the range).
Since there's a bit more of the metals involved in making the cartridges, the cost can be a bit higher (more copper, brass and lead, by weight, after all).
There's nothing particularly "wrong" with the .40 as a defensive caliber.
Personally, after having invested the time to send some many tens of thousands of .40 downrange, I've found it's made me a better shooter. By that I mean it's made shooting 9's (even +P & +P+) and .45's seem easier.
Over the long run, I think it offers a nice compromise between a heavy 9mm (147gr) and a light .45 (185gr) for those folks who like to use heavier bullets.
The expansion often demonstrated by a good .40 JHP might exceed that of a good 9mm JHP by a couple hundredths of an inch, give or take. Maybe the same amount of diameter less than a good .45 JHP that expands. If that matters to you, go for it. (Think dime, nickel & quarter, more or less, but it's not like it's any sort of hard & fast "rule".)
Capacity? Less than similarly sized 9, and more than a similarly sized .45. (Notwithstanding that a .45 typically requires a larger grip girth.)
Lots of "average" shooters seem to find they prefer the felt recoil of a 9 to that of a .40 ... and some claim they can't notice the difference.
Well, as long as the holes in the intended target are appearing where they need to appear ... and the claimed "unnoticed" felt recoil proponents don't suffer any loss of actual accuracy ... pick whatever you wish.
I like that the .40 has heavier bullet weights available compared to the 9.
Don't lose any sleep worrying about the practical difference, though.
Pick what you like, for the reasons you like ... and if one comes in a prettier color finish, so much the better?
I just ask myself if I can put the hole where I want them to appear, as rapidly as I want them to appear (may need for the to appear?) ... and have no immediate plans to divest myself of any of the 3 major calibers.
