James Symington , Feb 05, 2008; 07:51 a.m.
I have a set of Lee grads and lots of adapter rings and am very happy with them. The adapter rings come in standard, wide angle and even super wide angle flavours that help reduce or eliminate artificial vignetting on wide angle lenses.
What I don't know is if Cokin produce rings that will allow you to get around this potential problem - somebody else with a Cokin kit should help there. It may not be an issue with the 18-200mm but it may become problematic if you get a 12-24mm for example.
What I would say is that a hard 2 stop filter is in my experience much more useful than a soft one for landscapes although of course that has its uses too.