I'm looking at purchasing an ND filter, mostly for waterfalls. How many stops would you recommend for this use. I often find myself at f/22 to get below 1/10 s, sometimes wishing to be able to get longer exposures. I think both 3 and 5 stop filters would be useful to me but need to choose just one at the moment. A variable is definitely out of my price range.

Water starts to blur at anything slower than 1/60th of a second, but I agree with you, I like to have even slower speeds. I prefer speeds of 1 second and slower and also like to stay at f16. I would go with the 5 stop. Using the numbers you listed(or close to it) opening up to f16 would make it 1/20th, but for simplicity sake call it 1/15th of a second. Five stops would make it a 2 second exposure (1/8th, 1/4, 1/2, 1 second, 2 seconds) that should slow it down really well for you.

Thanks for the input. I think I'll give a 5 stop a try.