I bought and use Breakthrough Photography filters, I think I have 5 or 6 of them and I have been happy with their performance, but I refuse to register them. What a horrible business practice. "Our name is all over the thing, but we will only cover damage to our filter if our filter breaks from manufacturer defects but only if you have registered the filter through our company and only within the first 30 days after purchase." SMH. It's a 25 year warranty that will not cover damage done to the camera or the lens caused by their filter, only the filter itself so unless you're dropping some serious coin on said filter, I see it as a waste of time. I like buying from and supporting USA companies, but what a ridiculous warranty!
@
hark
Use caution when spinning these filters into place, don't over tighten. The difference between X2 & X4 is not just in the layers of protective coatings on each side of the filter, the X4 is made out of brass and the X2 is aluminum. Brass threads will not stick, aluminum has a tendency to stick and can be impossible to remove. I've had one X2 get close to becoming a permanent fixture on a lens, but I used the ol' fat rubber band trick and a bit of patience to get the dang thing off!