david: same Hitech filter as star and 阿瓜. I bought the 3 stop hard edge and 2 stop soft edge and I'm thinking of getting at least a 3 stop soft edge. I think in many cases you need 2 stop + 3 stop (that's what I did in 52-55).
You can get the filter and holder from 2filters.com. Get the holder for your current wide angle lens (e.g., 77 mm for 17-40L)
For landscape photos, I think GND and CPL are the most valueable accssories and a must have.