I have been photographing for many years but have not really been a believer of filters until quite recently. Now, after having used them quite a lot this summer, I feel as if I've lost many years of good landscape shots and it makes me a bit sad. Now I use them all the time, I would say 9 out of 10 landscape shots, easily, and I most often combine the ND with the GND.
I use the Lee system, since I find it to be better than the Cokin in many ways. Their wide angle adapter ring lets me shoot at 16mm (FF) (the Cokin P-system let me shoot at about 21mm) and the "glass" is of better quality and does not tint the image like Cokin glass does. (There are of course other systems too, but these were the two I looked at.) I have a 2 step hard GND, a 2 step soft GND and a 3 step ND, which seems enough for now.
Are you looking to capture sky detail without darkening the foreground and without using multiple exposures, GND filters are a must and you will see a large improvement in your landscape shots.
Good Luck!
