You didn't mention it, but I thought I'd put in a plug for Singh-Ray's Vari-N-Duo. It's a screw-on ND filter and polarizer that varies from 2-1/2 to 8 stops with a twist of the wrist. it allows you to focus/frame, then increase the ND to your desired level, and then expose. It's more expensive, but worth every penny. If you don't want the polarizer, they have the Vari-ND which is essentially the same filter without the polarizing element. They also offer an add-on to bring the density up to 13 stops (I forget the name of the add-on). Professional quality (and a price to match, but again, it's very much worth it).