I've always had two problems with magnifying aids. First - when I use them I tend to lose my depth perception. I find that, for instance, I can't judge the distance between a paintbrush and the object I'm trying to paint with it. That problem, I imagine, can be cured with sufficient practice.
I once brought this point up with a gentleman who worked in the model restoration shop at Mystic Seaport Maritime Museum. I noticed he was using an Optivisor (or something similar), and asked him if he'd ever had trouble judging distances with it. He laughed and said, "That's never been a problem for me. I'm blind in one eye."
My other problem is harder to solve, but I suspect it's a common one. My right and left eyes are different. The best an Optivisor or other tool of that nature can do for me is to bring one eye into focus; for both to focus on the same spot, the right and left lenses have to be different. (As I understand it, virtually everybody's right and left eyes are different.)
When I was younger, I could build models with no magnification at all; I had trouble recognizing people from across a room without my glasses, but I was in the habit of taking them off whenever I sat down at the workbench - or, for that matter, the typewriter. (People at the museum where I used to work used to joke that they could tell when Tilley was getting serious about something because he took his glasses off.)
Now I need bifocals. After a couple of years of wrestling with them at the workbench, I had the optician make me a set of prescription reading glasses - lenses that only work up close. (They blur anything further away than three feet or so.) I had the optician make them out of heavy-duty, shatter-proof plastic, so in some cases they can function as safety glasses (though I also wear bigger safety glasses over them when I'm working at the table saw or the lathe). Even with those glasses, though, I still have trouble seeing really fine details on a model.
The solution for me turned out to be a pair of "flip-up" magnifying lenses that clip onto my reading glasses. They have about +3 power (I think); with their help I can see just about anything I need to see. They cost, if I remember right about $10.00. The reading glasses put the right and left eyes on equal terms; the "flip-up" lenses add the same magnification to what both eyes are seeing. Come to think of it, I suppose an Optivisor worn over reading glasses would work, but it seems like it would be pretty cumbersome.
I bought my "flip-ups" from Woodcraft (www.woodcraft.com), but when I checked the Woodcraft website a minute ago I couldn't find them. I'm sure such things are available somewhere, though.