I've always considered the 99 series to be a very good quality, reliable service-type pistol which has been somewhat under-rated in this country.
For whatever reason, Walther simply doesn't seem that interested in aggressively advertising the model line (which is imported for them by S&W). They do have a lot of activity when it comes to LE/Gov/Mil sales outside of the US, though. Maybe that's where they prefer to focus their attention and interest. (As opposed to Glock, for example, which we were told during my last armorer class receives 80% of its total sales and business from the US.)
I'm probably somewhat biased in my appreciation of the 99 series due to having carried an issued SW9940 for a few years, owning a similar SW9940 and a SW999c, and also having been to 3 SW99 armorer classes over the last 10 years.
Yes, there have been some revisions and changes in the 99 series as time has passed, including in the frame. That can pretty much be said for all the major firearms manufacturers.
I've handled a SW999 that experienced a cracked frame due to an ammunition "over pressure event" (case head blowout), and I've been told of another one occurring at another agency. The cause of that other cracked frame was considered to have been a barrel obstruction unnoticed by the shooter (SW9940) which resulted in the predictable over-pressure event. The crack wasn't easily visible, but each time the gun was fired the frame flexed at the crack and pinched the shooter's hand, causing him to eventually bring it to someone's attention.
Plastic frames are not metal ... obviously. They can chip, crack or break if dropped (especially in very cold environments) and they can't be expected to withstand the forces which develop in over-pressure events in the same manner as steel or aluminum frames. Big surprise.
I've fired more than 10,000 rounds through my SW999c (9mm compact in the AS, or traditional double action model), and not quite that many through my personally-owned SW9940. I've fired many thousands of rounds through my issued SW9940 and some other training SW9940 guns.
A friend has 3 SW99's through which he's fired many, many thousands of rounds. His pair of full-size SW99's have fired more than 55K+ each last time I checked with him. His newer SW9940c has only seen several thousand rounds. I keep him supplied with recoil, striker & mag springs.
Given my druthers, I prefer the 99 series chambered in 9mm, whether it's the Walther or licensed SW99/990L version. (Which is the same way I feel toward Glocks.)
I've had to repair a number of different SW99/P99's due to various minor issues ... bent slide stop lever springs, broken ejectors, weakened extractor springs, damaged rear sights (and/or sight base plunger), cracked slide end cap, etc.
FWIW, in the SW99/990L's the frame and frame parts are made by Walther, and the slide & barrel are made by S&W. (Meaning that except for the slide and barrel, everything else in the SW99/990L slides - striker assembly, extractor, sights, decocking button, etc - is made by Walther. The magazines are made for both company's models by Mec-Gar.
The only thing maybe a little disappointing about the P99 is that Walther only offers a 1-year warranty. S&W offered their lifetime warranty on the SW99/990L's, including the Walther-made parts. Go figure.
Walther parts can be expensive. For example, if you experience a broken ejector (which is molded into the housing block), the sear housing block has to be replaced, and if it isn't covered by a warranty period the retail cost of the sear housing is approx $100 (plus labor, tax & any shipping).
The newer P99's have some nice features not seen in the earlier models. If I hadn't been able to buy my SW999c for less than the Walther P99 9c AS, I'd have happily bought the Walther compact.
The P99 compact is a nicely balanced compact pistol, not much larger than a G26.
Good gun.
Just my thoughts.