When I decided to get back into firearms I did a LOT of research about muzzle energy, the flawed Marshall-whatever data regarding effectiveness, and read a lot of gun rags.
I decided on a .40, as it seemed to have almost the effectiveness of a .45, but with 2-3 more rounds in the gun than the .45 1911 [I'm in california: some had more, out of state]. I decided that 10 rounds of 9mm or 10 rounds of .40, and the .40 would be a better self-defense handgun.
I'd been away from firearms for 12 years or more, and it wasn't a regular thing before that.
I bought a S&W Model 410 [what I could afford]. I couldn't get a group size under 2 feet at 15 yards. I developed a HORRIBLE flinch. I started 'hankering' for something that felt better.
I wound up buying a .45acp [ruger P97: it is still surprisingly reliable and accurate] and a 9mm [baby eagle], as I had the money.
I was more accurate with either of them than the .40.
I sold the .40 in 2002 and kept the other calibers, expanding to include .357mag and .22lr.
I still had a horrible flinch that it was hard to compensate for, but my group sizes were now closer to 1 foot at 25 yards.
I only had outdoor ranges to use, so no ability to work from 7 yards and out.
Then I took a 5 year break due to moving/marriage/kids.
When I went back, I used my .22lr a LOT and then the 9 and .45 and .357, all at an indoor range starting a 7 yards.
My group sizes are now getting respectable, but what I found was that I shoot 9mm and .45acp better than .40.
I've recently picked up a .40 for a great price. I find that I shoot it with acceptable accuracy, but not as comfortably as 9mm, .45acp or my magnums. Ironically, I am more comfortable with 50 rounds of .44mag than with 50 rounds of .40s&w.
I agree that anything is better than nothing. I agree that it is better to hit someone with a 9mm in a critical area 8 out of 10 shots, than to hit them in a non-critical area [at best] with 4 out of 10 shots from a caliber that is too powerful for you. I believe I am more accurate and can recover my target/sight picture for a follow-up shot much easier on 9mm than .40 or .357mag [or .44mag]. I am faster on second or third shots on the 9mm than the .45acp, but not by as much of a margin.
I think that, for me, in california, 10 rounds of 9mm are not a major reason to skip 8 or 10 rounds of .45 [or of .40]. If I lived in a 'free' state, where I could have 16 round mags for my Baby Eagle in a carry gun, I might favor that over a 1911 with an 8 rounder [or S&W M&P with 10 rounds], but, comparing 10 rounds of .40 or 9mm to 8-10 rounds [1911 or M&P.45] of .45 and it is about shot placement to me.
If I can't hit it, I shouldn't carry it.
I will say, if I had a CCW, I'd have a 9mm for summer use [lighter clothing I am wearing, as well as bad guys], probably a .45 for winter, and a .357mag for trail hiking [cougars, etc]. Even though I'm better with the .40, I still find it the caliber I am worst with.
I'm no caliber snob, but I was for a while. Now I am biased in favor of what works for me on a regular basis. And, with my hyper-extended left wrist, 9mm left handed [single handed] is controlably, but the others are harder to control with my left hand.