Interesting reading guys. I have or havr had a few of the filters mentioned here. One thing I would say is if you want to use grads then stick to hitech or lee as you will sharp get sick of trying to run through this process if screwing on a 10 stop then putting the lee or hitech slot system over it :/ One good idea is if you own the above systems and want to use grads go for a 6 stop as you can still compose with it on. I have a Hoya nd 64 which I put my lee kit on after. At sunrise this is great. In daylight I even add my hitech 10 stop over the Hoya 6 stop. Sometimes even in bright daylight 10 just isn't enough. For this I'm looking into the minutes rather than seconds to give the sea a flat milky appearance or turning clouds into full on streaks
Check out a guy called Joel Tjintjelaar on Flickr or 500px and look at his fine art mono stuff.