Can't give you the general impression of the .40; only my own opinion. When I decided to purchase a semi-auto for self defense, I read a lot about different calibers and came to the conclusion that the .40 was the way to go. Too many friends in law enforcement who jumped on the "wunder nines" and then found them inadequate to the task. Remember talking to a State Trooper neighbor way back when who spoke of several instances where the nine just didn't get the job done.
I checked out the history of the .40's development and thought it sounded like a great cartridge. Best there is? Naah, probably not. The .45 has a great reputation (local city cops carry .45s), the 10mm is a real brute, etc., etc. But the .40 seemed like a good step up from the 9. Today, there are lots of different factory loads in .40 and although it isn't as cheap as 9mm, the price has come down.
I shoot a Glock 23 and a 27, as well as a S&W 4013. All have been totally reliable with the .40. I've had no second thoughts, and recently sold my last remaining 9mm handgun to cut down on the number of different cartridges I need to stock. No second thoughts here.
Shoot straight.