Upload & Sell: Off
400 do is a very interesting beast. When I was researching it, I called canon and the rep explained that once a 400 gets out of alignment , even service center cannot bring it back to 100%. He suggested to carefully evaluate if buying new and avoid buying used?
Given that I would not borrow one.....
I have no idea where that kind of crap comes from, but I suspect it is the kind of misinformation that, when repeated a few times, gives birth to internet myths. My experience is that the 400 DO is just as likely to be capable of being properly serviced as is any other lens.
To the OP, I don't have as much faith in micro adjustment as some others do. Precise results require absolutely perfect positioning of the camera in relation to the focus target. If the height is off by just a bit or if the plane of the sensor is not perfectly parallel to that of the focus target, the results will be off. I've tried various methods, both home-made and commercially made, and I've seen that either can produce inconsistent results. Because it is possible to drive oneself crazy with micro adjust, I've finally come to the position that I don't worry about it until/unless I see consistent front or back focusing in real world pictures. If I do see either, then I do the best job I can of making a micro adjustment, and I judge the results of that adjustment by seeing if, in real world shooting, I'm still seeing consistent front or back focusing. I suppose that the good news is that, if you are off by a few adjustment points, the effect when looking at actual prints, not screen images at 200%, is negligible.