Anyone else using freeform magic in your Magic World game? If so what rules are you using? What GM challenges have cropped up?
As for me we use our own house rules The Second Way which is a take off of deep magic. My biggest challenge so far are the player's creative use of magic to solve problems in general and the availability of gate spells in particular.
One of the magic systems I use is the Maelstrom magick system. It is elegant, freeform and subtle, and encourages strategic and situational thinking by the mage player. It is particularly good for low-magic settings.
The Maelstrom magic system
There are no specific spells, just one generic Magic skill. Magic is situational and based on probability of an event occurring. When a mage wishes to cast a spell, the referee gives it a level of difficulty:
1.