Yah, there has been a fair bit said about these pistols... even in the last couple months. Check those out for some good info.
I have a P99 AS in 9mm. I like it a lot. It has basically become my main centrefire pistol - just because I get better results with it than the other guns I have.
The striker-fired DA/SA design of the gun is very unique. A striker fired gun with a decocker? I can't think of another one. The 5 interchangable front sites are a great idea - I wish every gun came with something like this. The interchangable backstraps are not a big deal for me, the ergonomics of the gun are excellent no matter which backstrap you put in.
The gun has its detractors but I have shot mine
a lot and have not had problems. No fail to fires or fail to ejects, and the accuracy is more comparable with a SIG 226, HK USP or Beretta 92 than with a Glock, Steyr or XD-type gun. Some people also like to put forward this idea that the gun is somehow less durable than Glock, SIg, etc. Owning and shooting this gun and a lot of it's competitors, I really don't see that, but everybody has an opinion right?

I know that the P99 beat the USP for a big contract for the Polizei NRW over in Germany. Clearly durability is going to be an issue in that decision. The Montreal PD use them as well.
In saying all this, note that I absolutely
LOVE Glock, SIG, HK and Steyr pistols. I'm not a devotee of Walther and nothing else. I just think that Walther is every bit the gunmaker that their more ascendant German and Austrian breathren are. They have lots of good ideas and they make really nice kit.
The only negative thing, and I've mentioned this on the board elsewhere, is that my girlfriend and another woman that was shooting this gun felt that the felt recoil was pretty heavy for a 9mm. My girlfriend doesn't like shooting the gun because of this. Personally, I just see the gun as more "snappy" than anything. It has more of a sharp "crack" sound to it when it fires than other 9mm guns, and you feel it in your hands after shooting a lot of rounds in a row, but it is still pretty tame. It's a 9mm after all.
On the barrels, for the rest of the world it is 4 inch (102mm) for the 9mm and 4.17 inch (106mm) for the .40 cal. For Canada, both of them are 106mm. Walther has a long history of accomodating dumb Canadian gun laws with the P99 (but unfortunatley not with the P22 or PPK). 106mm versions of the P99 have been coming into Canada for years and years now.