Ade is one of the few ZigZag users so far to put their impressions into words (and pictures) so this article makes very interesting reading.
One of Ade's observations is the compactness of the ZigZag. But the small size can present a problem for users wishing to terminate their climbing line with a knot. Apparently the bulk of the knot can interfere with the functionality of the 'zig zag' friction links in some configurations.
This particular problem is not unique to the Petzl ZigZag - see a previous blog post here regarding a similar issue with the DMM Hitch Climber set-up.
According to Ade there were no problem with the ZigZag in foot ascender mode. But when a second pulley was introduced to the system (for increased mechanical advantage) the action of pulling down on the trail rope tended to close the gap a little more than it should and this had the tendency to impede the locking action of the friction links.
More 'issues' are bound to surface once the Petzl ZigZag gets into the field. At this early stage its doubtful climbers will loose too much sleep over this one - 99% of all climbing lines leaving Treetools are spliced or stitched in any case.