well, I used to own a P99 in 40 S&W--it was one of the "all german" ones they used to make a big deal about making sure you got when they first came out. Let's see...
1) How reliable are these guns?
Mine was totally reliable with any kind of ammo I tried--never a failure of any kind that I recall
2) How good/bad is the trigger?
meh, the trigger was about on par with most service type handguns
3) Anybody had issues after the warranty expired? I'm wondering if Walther/S&W would fix for free after the 1 year?
I'd look at the walther forum and see if people there have made claims after the warranty period and if they paid. I guess it would be at their discretion. I have to say that a one year warranty is not super impressive given the number of manufacturers who offer lifetime warranties. But I wouldn't really expect any more problems with the P99 than any other similar pistol. Stuff happens and things fail, though, and with many other manufacturers you'd betaken care of.
4) How well do these carry for CCW.
about the same as any other pistol of it's type, by which I would mean Sig Pro, Glock, XD, etc
There is nothing wrong with the P99--decent ergonomics, weight, easy takedown, very reliable in my experience (though he accuracy of mine was nothing to write home about). From my point of view though there is little reason to get one when an XD is cheaper, mine have all been more accurate than the P99 I had (this is a sample of one, though, for P99's--others report theirs as being super accurate so some must be), just as reliable, just as easy to take down, similar size, weight, profile for carry, and I assume Springer's lifetime warranty? Never specifically checked that but they have a lifetime warranty on everything else I think and are known for excellent customer service. If you've got the P99 itch, though, may as well scratch it
It's not like you're likely to get a BAD gun.