peevee1: Does f/6.s mean that no AF tracking will work on any DSLR because only central AF point will work with this lens, and on some DSLRs even that central point will not work?
The f/5.6 specification advertised for most DSLRs is *not* a hard limit, at all. An f/5.6 lens will give you a good AF experience in most conditions, so that's where they drew the line in the sand. The difference in AF reliability between f/5.6 and f/6.3 really isn't noticeable.
Poor light, poor subject contrast, and smaller apertures all increase the likelyhood that AF won't be reliable (i.e. it'll repeatedly hunt instead of locking on). You can have problems focusing on a clear, featureless sky with an f/1.4 in broad daylight, due to lack of contrast. You can have problems focusing with that same f/1.4 in candlelight. You can get autofocus lock-on in good conditions way out to f/10 (I've done it with a Celestron C5 + AF confirm chip) and it's even worked a couple times for me at f/13 (an f/6.3 with a 2X teleconverter).