Going to correct some items here:
Here is the Walther Page on the P99AS:
http://www.waltherarms.com/products/handguns/p99/
and here is the Walther Page on the PPQ:
http://www.waltherarms.com/products/handguns/ppq-m2/
One can see the P99 AS actually has a LIGHTER trigger than the PPQ, as described by the manufacture. They both reset at the same point. Therefore, if the PPQ is excellent for the trigger being smooth and light...the P99AS is above the PPQ.
The rear sight, as mentioned, is the biggest issue. It is locked into place by a plunger. Truly a none issue in normal use, but this is the one fault of the P99.
The Walther P99 was the FIRST gun with replaceable backstraps. That would be Glock and HK taking from Walther, not the other way around as suggested.
The Walther P99 is the FIRST DA/SA with decocker striker fired handgun. It completely mimicks a DA/SA (think Sig, Beretta, etc), but also provides a full forward trigger in single action if desired.
Some feel the P99 is a bit snappy. I agree. However, the grip and slide profile allow me back onto target faster than a less snappy gun (Sig, Glock, etc).
They hold their value well. They are a bit bigger than their comparisons (G19)....other than that, I love them. About to pick up my 4th P99 after having sold a PPQ M1.