I've seen those before, never used one. It's a simple "L-pad" Design. If the components are good, then it should be fine. If not, not so much.
I bought a pair of Shure switchable in-line attenuators from Full Compass, and they work fine. They're just fixed resistors rather than a pot, which may be better for long term reliability.
I have an H4n as well. The input is sensitive.