Overall I'm happy with my Walther P22, I agree you shouldn't have to "FIX" your own pistol for it to operate correctly but to be honest I actually enjoyed breaking it down and taking it apart to fix it. I don't have any experience with the Sig Mosquito but have read reviews of it having problems also, maybe not as many but I've heard of both having issues. Below is my target at 20yds, I got most of them on the 8" target but had a few on the cardboard. I took the Walther P22 out to my wifes parents house today and shot 90 rounds through it at about 20 yards and did pretty good with absolutely no more problems/issues as before.
Below is the link to the Walther P22 Bible and my before/after pics of the repairs/mods to the pistol which are not the best but detailed pics are inside the PDF which give you a better idea of the work being done. I did some research online and found a PDF called the Walther P22 Bible which is a detailed step by step owners/service manual to fix the problems yourself. After buying my new Walther P22 I kept having problems with the cartridges getting stuck on the feed ramp/barrel chamber, hammer getting stuck on the slide/saftey pin and last issue was the trigger bar ears were taking chips out of the underside of the slide rails because the ears were sharp/jagged.