I have had mine for a little over a year and have put more than 1000 rounds through mine at the range.
Accuracy for pistol ranges is good, in my case, but by no means match grade. From the test target I got with the pistol, at 25 meters it was shoot ~3" moa. My own shooting skills are not so good, but 2" at 10 yards at slow fire are normal.
The slide lock problem was fixed with new magazines. You can tell by the colour of the followers in the magazine. If they are black or orange, they are the old style magazines. If they are light or dark blue, they are the new magazines that correct the slide lock issue.
If you happen to pick one up used and you have the old magazines, you can send them to Walther USA to have them replaced with new mags for free.
The trigger pull in double action is *long*... A little too long in my opinion.
As for single action, it is short, crisp, and to the point. This was a bit disturbing at first because it would break before I though it would. This came from having shot Glocks primarily before picking up the P99, which have a slightly longer trigger pull in SA.
Just weighing in with another opinion, hope it helps.
I would certainly look at trying one before buying though. Ask around to anyone you know who shoots and try to pick one up. I wound up being happy with my purchase and the price, but would have been a bit ticked if I spent that kind of money for something that didn't perform as the P99 does.