I have the Walther. No problems at all. The newer models and the S&W versions had a standard rail, most of the Walthers have an odd-ball proprietary rail. If the rail's important to you, get the S&W or a later model Walther.
The gun is reliable as all get out. Easy to shoot in 9mm. I have no experience in the .40 version.
As you've notice, they're very comfrotable in the hand, and the switchable backstraps allow adjustments. It comes with three different backstraps. I like the grip angle far better than the Glock.
The gun produces good accuracy. Not as good as my P7M8, but about the same as my CZ-75/Sig, and just less than my Beretta (but only because I've been shooting the crap outta that lately). Certainly as good as anything out there for the same price.
The controls are easy to use. The striker de-cock is easy to get to and use on the da/sa models. On the other models you don't use it anyway except for disassembly. The mag catch is the same as on the P7/USP and can be operated by lefty or righty easily. The striker protrudes out the back when cocked, so it will indicate a cocked condition by sight or feel.
I have the green DA/SA model. DA is easy, SA is not bad either. The trigger has a different feel though. It probably won't bother you much if you've shot revolvers, but if some people just don't care for it and shoot the gun poorly because of it. I like the trigger, but it is hated by others. I'd say if there's one feature you need to "try before you buy" it's the trigger. If you like the trigger, the gun will be great. If you don't like the trigger, the gun will suck for you. This is really what would define if the P99 is right for you or not. If you can, shoot one before you buy.