Firstly I have to state that I know the Lee people wel and do some work for them, so my comments are slightly biased perhaps, but the Lee system, although expensive, is excellent. One comment I would say about their adaptors is, don't buy the wide angle one if you aren't going to use it on a wide lens, as they are considerably more expensive than the standard one, however on the wide angles they help with vignetting, the one lens that even their wide angle adaptor won't work with is the 16-35 2.8 L mk 1 and 2 , it vignettes when you go wider than approx 20mm. (same applies for the 17-35)
I get round this by simply holding the ND against the lens without a holder.
George