It is a wonderful gun. Do a search on "P99" in the the forums. You will find dozens of threads from all the P99 lovers on this board.
Any problems? Just one I keep having to go to the store to buy more ammo, the gun keeps seducing me to shoot more.
The pros are it is light weight, it's a Walther, it can be fitted to practically any shooter thanks to the multiple sights and back strap inserts, it is tack-driving accurate. The cons are the above mentioned ammo problem and as soon as you shoot one you want to go out and buy a second one.
Mine has been 100% reliable through the first 1,000 rounds with a mix of ball and +P hollowpoints. Some of the folks here have way more than that through theirs with the same results.
Mine is the 9MM by the way. There were some problems with the first 3,000 or so .40 S&W models, but the newer ones with the German proof marks on the slide have been very good.