TO be honest, I looked at the Harbor Freight ones, they had a couple options on the website, but the local store had only one option, and the optic was a plastic and the focal point was very close to my nose.
Fastforward to the LHS, actually a train place, and they had every model optivisor, I settled on a #7, I think it's a 3x, the lower numbers were not enough, the #10 was far too much, and again the focal point was very close to my eyes.
I noticed that you can order the other lenses and switch them out, and they are glass lenses. I folded some teeny photoetch, and let me tell you, I can't believe I waited so long. It's GREAT.
I paid about $40, I consider that a bargain just in the time I already used them.