I've been using an Optivsor with a No. 5 lens for quite some time now. It works great. Wish I had known about it long time ago as it lessens the strain on the eyes, good or bad.
As for price... in this case, I believe more expensive is not necessarily better. Mine cost me somewhere around 18 pounds and I have had it for a number of years now. You are paying for the lenses, essentially, and if you look at all the other (more expensive) options, the plastic head piece are all pretty much the same. The lenses also all look exactly alike, expensive or cheap. But hell if you want to pay 150 pounds for the same thing as one for 18... go for it.
I recommend an Optivsor for all hobbyists. Protect your eyes!