To my way of thinking, it adds SOME protection to lenses that don't offer it. As to the amount of protection, that part is a bit unclear, but something is better than nothing, right?
I'm not convinced that it's any protection, sort of like driving through a car wash with one window up and three windows down – the inside of your car is still going to get wet. In fact, some may be worse than none.
Dust Donut installed on the Canon 60D with EF-S 17-55mm f/2.8 IS lens
...Installing a Dust Donut to your unsealed lens offers a convenient, seamless solution some of the most harmful and expensive problems that threaten DSLR users.
Let's see...a camera body with limited weather sealing on its many switches and openings, an extending zoom lens that costs more than the camera with no sealing on its switches, zoom barrel, and actual vent holes at the front element, and they're telling me their product is a 'solution'? To the extent that someone actually believes
that little rubber ring provides protection from water and goes out shooting in inclement weather, that's two pieces of gear potentially destroyed for the price of a cheat piece of rubber. What a deal!
Don't get me wrong – I think it's a great idea for certain lenses. It just doesn't have the broad applicability to all lenses as they seem to be promoting. Personally I would certainly consider getting one of these for my 100-400L, because that would complete
the sealing on that already mostly-sealed lens. But there are only three current lenses for which that situation applies.