I have about 15 hours on my new 8700. What I have learned is that you need to:
1) adjust the left /right positioning first.
2) position the image vertically so that it is above where you would actually like it.
3) lower the image so that it is only inches from being in place.
4) finally, lightly tap the zoom/focus rings downward to remove any slack.
Actually, the projector I received is pretty good at holding its position, and in the 15hrs of use I have never noticed it shifting on me. However, it is shelf mounted against the back of my theater, and not ceiling mounted, although I don't think this would make much of a difference.
Like most owners of the 8700, I find the picture this projector creates is absolutly amazing, and feel it is worth the extra effort the lens shift wheels create.