CF Cap is 12% from items, + 5% from GM Scribe (may have been increased to 10%), and I think up to 4% from bard masteries.
My parry mage works just fine with it. Though I think for PvM there are better properties to go with. For PvP at 17% thats a 1 in 6 chance to proc, which provides a little bit of forgiveness to mages for spell timing.
It doesnt save you from having to time your spells properly, but when you get hit .25 secs before your spell would finish and it goes off, you definitely feel its worth.
I also use a resonant shield with kinetic resonance. The dci isnt as good as some items, but its another chance to not be interrupted.
CF won't save you from interruption if you get hit above 20 damage (I think is the interrupt threshold). But thats quite enough for you to occasionally shrug off poison/bleed ticks, lightning, fireball, velocity, and magic arrow procs/hits and keep casting.
