I never tried a P-40 but have exchanged a lot of emails with folks who have.
Guys who have some experience with handguns, and with the P-11 in particular, seem to like them pretty well. But an awful lot of folks bought them as their first gun.
The Kel-Tec P-11 and P-40 are not the easiest guns to shoot well, and limp-wristing can be a problem. The long double-action trigger is an issue, too.
(I had a P-11 for several years as my main carry gun. I shoot other guns pretty well, and could shoot the P-11 well when punching paper, but in an IDPA match, etc., I didn't do that well with it.)
The earliest P-40 has a bunch of problems, resolved by the members of the Kel-Tec Owners Group -- most of it was a modification to the feed ramp on the barrel.
There were so many returns of the P-40, for the reasons stated above (primarily inexperienced shooters) that Kel-Tec just stopped production. They're in business to make a profit, and a gun that isn't profitable is bad business.
If you're patient, you'll probably find it a good gun. But be prepared to practice. (But that's probably true of the similar Taurus Millenium guns, too.)
If you haven't been there yet, try visiting
http://WWW.KTOG.ORG