Recently I purchased the Kase Wolverine Magnetic Circular Master Kit and I am just loving it. I purchased it from Kase Filters Canada website and Jim Brompton a Canadian ambassador for Kase. Jim is a great person and accomplished master photographer. I haven't noticed any colour shift or decrease in sharpness.
Right now I am trying to time my outings with different weather and wave heights to learn the combination of filters and exposure times required to get the effect desired.