I use Helicon and the RRS B 150 focusing rail. I also use the Novoflex Balcan-AF below, which has a similar rail function in addition to the below. They are both manual, but for my use I cannot say I have had real accuracy issues. If I increased magnification beyond what I do, I´m sure that would change.

I must admit that I don´t do this very often, but in general I am quite pleased with the results. The only problem is perspective. Moving the camera flattens the perspective, which in some cases ruins the picture. If anyone can provide a way to fix that (my photoshop skills sucks), I would probably do it more often.