What do you think of the Lee system? I've been looking at it with great interest.
I really like it. As I said, I'm using a 16-35mm I on a 5d MKIII and I'm not getting any vignetting. Similar to all the forum posts and reviews I read, hold true. I am using the 77mm wide angle adapter which is part of the reason. I have only gone 2 ND grads long from the lens. I haven't needed to put on a 3rd. I do plan to save up for the 105mm polariser. that I will mount on the end. I've heard it's big enough not to cause an issue either.
I am using Lee resin filters with it. The build quality of the holder itself is quite nice. Slides around your lens nice and smoothly and holds the filters snug.
So that's a long, I definitely recommend it. I think you will be happy with it. If you do get it, I recommend picking up the lens cap for it. They're $5 or so. It's just a plastic cap that you put over the adapter, so when you're say walking around a lake shooting from different locations, you don't have to keep unscrewing the adapter, you only have to quickly pop off the holder and put the cap on.