I've been using one for a while now and it works fine, but one thing I'm curious about. The grain (at least that's what I hope I'm focusing on) is almost wormy looking, which is not what I expected. I'm sure it's the grain, because I can see the details are at their sharpest when the little "worms" are at their sharpest.
For 35mm I focus on top of the paper using the red safe light filter on the enlarger. For the 4x5 I use a throw-away piece of paper and no safelight. Once it's focused I change paper (remembering to turn the enlarger light off, of course).