In my experience, they are excellent as far as reducing glare. Much better than a sunshade. The only downside is a slight decrease in clarity when looking through them. I'm not sure about the threading. SWFA's SS scopes are all threaded for them, but I'm not sure on any other brands.
Depending on the scope and if it has a sunshade that it'll work with, USO can put one together for you if you supply the sunshade. They put one together for me for a Leupold scope, looked good and worked great.
Made several for my own rifles. It's not hard if you have some DIY skills.
I did an "DIY ARD" thread last spring. Just made another one last weekend for a guy on my department to a slightly different design. They are easy to do and you can have some fun with design.
When I do one, I not only try and reduce reflection and glare, but eliminate the "round black circle" that is easy to spot.