I carried the S&W version, the SW99, chambered in .40 S&W as an issued weapon for a few years, and I own a couple of SW99's, a standard size .40 S&W and a compact 9mm. I've been through the SW99/P99 armorer class more than a couple of times and have developed some basic familiarity with the standard models.
Good design and well-made pistols. Decent for service-type use.
The P99 series is a popular Walther product line outside the US, too, with much of Walther's production capability being focused on meeting demand outside the US for the time being (according to Walther America/S&W).
S&W is very good about taking care of Walther customers. I've heard of many instances where they handled a repair issue for a Walther customer after the Walther 1-year warranty had long since expired. (FWIW, S&W extends their regular extended warranty to purchasers of their licensed SW99/990L models, even including the Walther produced parts.)
Walther parts are sometimes a bit difficult to get, if the parts are back-ordered from Germany. I've had to wait for Walther parts from time to time.
Felt recoil? 5 folks will likely give you 5 or 6 different opinions, since at least 1 of them will change their mind from one magazine load to the next.
I happen to think the 99 series is at its best when chambered in 9mm, but I feel that way about other pistols, as well, including the Glock ... but that's nothing more than a personal opinion.
The 9mm version does offer many folks the same advantages in the way of better controllability and recoil management. I easily decided to go with the 9mm compact after handling and shooting a couple of the .40 S&W compacts.
And yes, the 99 series was originally designed around the 9mm load. When S&W and Walther started their partnership, S&W engineers suggested some changes to the frame for their licensed models, and designed their own slides & barrels. We were told they wanted to enhance functioning with the harder recoiling .40 S&W cartridge in their licensed SW99 .40 S&W, and then later revised the SW99's slide & barrel to optimize it for the lighter recoiling 9mm (according to what we were told by factory sources). In other words, they focused their efforts on the .40 S&W cartridge first for the SW99 version. The 9mm version later received some lightening cuts and other minor revisions in the slide for the 9mm cartridge.
One of the early results was S&W engineers discovering, via high-speed photography, that the magazine body & follower design in the existing .40 S&W model magazines was apparently involved in occasional premature slide stop lever engagement. This condition had apparently been reported with some early P99's chambered in .40 S&W even before S&W started making their licensed SW99 version, according to what we were told. The information was given to Walther and eventually used by Walther to have Mec-Gar revise the magazine & follower design to the present form. S&W later revised the shape of their .40 barrels to enhance unlocking/locking with regard to the clearance of the rear of the recoil spring assembly. Lots of minor changes and revisions occurred.
While I don't use my standard size .40 S&W model much, I do enjoy using the compact 9mm model for training & practice. It's become one of my favorite 9mm pistols for range use. The practical accuracy I obtain from it generally exceeds that experienced with either my G26 or 3913. For me, that's saying something.