I just finished running a game of Fate Core using one of the new magic systems presented in the Fate Core Magic System Toolkit. The system was called Stormcallers. Basically the players each had control over an storm element: Inferno, Glacier, Flood, Earthquake, and Thunder. we made characters and I ran them through a heist scenario. Over all it was a blast to play and the magic was very quick and intuitive. It was a thrill ride.
We did run into a few issues though. The first was the names of the various storm elements. It was inconsistent, which led to some debate as to what each power was actually called. Not a huge issue really, but still it was a bit annoying.
the other issue we ran into was that some of the elements were poorly defined in the fiction. I had assumed that Thunder would allow for control of the winds. But there is nothing in there saying that. It talked mostly about lighting and thunder. If it had been described a bit more I think that this would not have been an issue. We also ran into an issue with misunderstanding what a barrier is. Barriers are introduced in Stormcallers as a way to block movement from one zone to another. One of my players wished to use this a s defense, and as written it does not do that in the way he wanted(i.e. in the same zone as the baddies). I did not think that this is a problem with the rule itself, more in its perception.
If you wish to watch the game we recorded it on hangouts, check it out if you like: