Because I was so taken with the Apocalypse D&D rules and because my current 1st Ed AD&D group is so willing to let me lead them around by the nose, I switched out the AD&D system and swapped in Apocalypse D&D.
The party fluctuates in size between 6 and 8 players. We have the whole race-class spectrum covered pretty well. They're all between 3rd and 5th level. We run published adventures I found for $1.50 apiece at a local bookstore. It's silly, dumb dungeoncrawling fun with people who aren't traditionally "gamers". So, that's the background.
We set about converting the characters, which was pretty easy - though, to keep their expectations up, I did a few scratch conversions of things I knew they would expect - certain class and race features they'd come to enjoy and who may not have been fully represented in ApocD&D. Mostly stuff from Unearthed Arcana.
Generally speaking, the players loved the new rules. They liked having choices and that failure actually meant something. They loved the spellcasting moves. They loved that they got to dish out and take more damage than they were used to.
There was some attempts to "game" the rules when players found that they had to hit certain stats to level up. A lot of requests to Parley and Defy Danger just to gain levels. After I laid down the law ("stop that. no, seriously, you can't parley with everyone you meet just to get points") it seemed to clear up.
There was a little bit of worry on my part - the characters advance VERY quickly. I think they're likely to advance too quickly for my liking. They all gained a level in the session we played. Combat generally means at least five or six Go Aggro rolls for anyone participating, so if you're tagged for STR you never have long to wait.
There was also some confusion (mild and small though it may be) about how a backstab works. Is it "Making Your Move" or do you make your move, set up the backstab and then Go Aggro?
All in all, a pretty smooth transition but I wonder how it'll hold up to campaign play. TIME WILL TELL.