My two cents:
The construction of the Auto-Collimator (AC) is critical. It is the most sensitive of the tools and is used for final check/touch-up. The machining of the AC tube must be held to close tolerances so that it fits squarely in the focuser. The mirror must be perfectly square with the axis of the tube. Even 1/1000th inch of tilt in the mirror (like could be caused by uneven setting of the glue used to affix it to the tube) can spoil the accurracy of all collimations performed with this instrument. The squareness of the mirror can be checked by carefully collimating your telescope and then rotating the AC in the focuser to see if the reflections wander.
With the above information in mind, I would definitely avoid the "five-dollar" Auto-Collimators that keep popping-up at star parties. These things are usually made out of a plastic electrical box hole plug and a piece of reflective scotchlite type tape. The reflective tape is invariably wavy and off center. Also, this type does not actually reflect an image - just a general glow; so you really CAN'T use it as an AC!
If you are going to homebrew Tectron type tools, at least spend the $5.00 to get their book "Perspectives on Collimation". It's written in a preachy and pretentious style - but does have tons of good info.
Bob Haler