Just that I've only used them for landscapes with a relatively flat horizon so far. That's all. If I understand correctly, if you hand-hold a hard grad, while the shutter is open for longer exposures, you can feather (H uses the term dither above I think) the filter to soften the edge anyway.
Tried to find a youtube clip that showed the technique but can't find it at the moment.
H would be better at recommending the first one or 2 to get. The above is just based on my preferences. But if you're shooting different things, the advice would be different.
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