I've managed to use filters on my old Sigma 12-24mm and TS-e 17mm full frame lenses. I currently use the PhotoDiox Wonderpaner system. Yes it works, not it's not easy or nice to use 145mm circular filters. But it's great to have a Polariser option which can ben used fill tilt or shift. But, working with such large filters is a serious PITA out in the field. I also have a 3 stop and 5 stop ND (not grads) which are ok, but have a really odd colour cast to them. Functionally they work well, but stacked (ND and polariser) and the lens can't be used with it's fullest range of shift...but it's certainly enough for most situations. Pointing this lens directly into the sun unfortunatly yeilds an increase in internal reflections and flare...which isn't there with the native lens.
I have high hopes for the new ef 14-24mm f2.8 L, I'm sure it'll be stellar. Hey, we all KNOW it's coming at some point....it's just when. Canon are doing some proper developement these days and pro gear is taking a while to hit the shelves...but when it does...it's top gear.