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 .