I have been pretty satisfied with my .22 conversion kit for my P226 9mm. It is certainly as accurate as I am, and I like shooting cheap .22's out of the same platform I use for 9mm. I've only shot Remington Golden Bullet .22's, so I can't comment on ammo flexibility.
However, I initially had a few mechanical issues:
1. First, the .22 conversion slide rides (rode?) a bit rough. Especially in the beginning, I had lots of issues with it not getting back into battery after cycling. the slide would come forward but stop about 2mm short. I worked around it in the beginning by doubling the amount of oil I would normally use on the slide rails. After 3000 rounds through the kit, though, things seem to have worked in nicely.
2. I have had some issues with the .22 upper balking at going back on the pistol. It seems to require very precise alignment of the take-down lever. I've never had an issue EVER with the 9mm slide, but the .22 fights me sometimes. Also, every time you put it on, you have to manually push the .22 barrel back with your finger before you can rotate the take-down lever back to locked. It seems this is standard behaviour for this kit.
3. The spring guide rod in the .22 kit was badly chewed up after only 1000 rounds or so. I have a stainless rod on order from sigpower. (I also have a mag follower on order. I can let you know how that goes when it arrives.)
None of these issues would stop be from buying the kit again, but I thought you should go in with your eyes open.