The P11 has a "true" double-action mechanism, meaning each pull of the trigger cycles the hammer through, whether or not there is a round in the chamber that fires. It has a magazine that holds 11 rounds, and can accept S&W 59-series magazines that hold more. The gun is a little but wider than the PF9. It's trigger pull is substantially longer.
The PF9, which I carry, holds seven rounds in it's magazine, and is significantly flatter than the 11 ("PF" stands for "Pistol, Flat".) It's hammer, when in the ready-to-fire mode (such as when the slide has been cycled by firing or by chambering a round), is partially locked by the sear. Pulling the trigger releases it. If the gun does not fire, another slide-cycle is needed to try again. The trigger pull, because of this set-up, is much smoother, but the tradeoff is the lack of "re-strike" capability. Many of us find this to be a fair trade, given the relative reliability of modern centerfire ammunition.
I've had zero issues with my PF9, which I have owned since April of 2010. I am not the original owner; I don't know if the prior had any issues with it. Mine shoots a little low with the most-commonly-available 115-grain ammunition, about four inches at 10 yards. I expect it's sighted for 124-grain or heavier stuff.
Neither is an all-day range gun; they are too light to help much in taming recoil or muzzle-flip with the 9mm caliber. I enjoy shooting about fifty rounds max during each range session, and make sure I have other guns with me.
Mine is carried pretty much any time I am awake and not at work.