Haven't tried an OHP yet, but keen to hear of anyone elses' experiences as well.
The 4x5" projectors are very expensive.
So I have tried using the 35mm slide projector (as a light source) in combination with 150mm lens on 4x5 field camera (to hold the 4x5 positive), just to see if I could get a decent enlargement at home.
Got a pretty good size and focusing OK on screen, but as you'd expect severe vignetting and generally not enough grunt. Unfortunately haven't found a solution as yet (but will try anything .
Some years ago, during a career day at my high school, a local studio photographer used an overhead to show some of his 8 x 10 color transparencies. It worked fine and, needless to say, the kids were surprised to learn that he wasn't showing enlargements. Actually, I was surprised too, since I didn't realize anyone in the area was using 8 x 10!
Back in 1970's, I tried enlarging a few negatives onto what I think was called Kodak Fine Grain Positive. The stuff could be bought from Freestyle in rolls about nine or ten inches wide and processed under safelight like prints. It worked fine as an overhead transparency as long as the density was kept a little on the light side. It was hard to deal with, however, since it had a pronounced curl and had to be taped to the projector or mounted in a strong frame. I've looked for this film in recent years and had no luck finding any.
We do it at school regularly, some teachers like it, others hate it. It works well to show the image, but the optics of an overhead projector aren't that great, and it really doesn't do a large format piece of film justice at all; IMHO the projected image looks simlar to a 35mm slide projected, sometimes not quite as sharp, looks almost like a soft focus lens with some projectors I've seen.