I heard that halucinations take twice the damage as normal units. Are they different in any other way? Armor, speed ?
In addition to taking double damage, they also don't output any damage, which is probably obvious. I can't find any references to them having otherwise different stats such as lower speed or not benefiting from upgrades. (Edit: tzenes says in the comments they do not benefit from armor; I haven't tested it.)
According to this site, they are destroyed instantly by a psionic storm. Also, detectors can spot them, so I'd guess if a detector was in the area their priority for the AI to attack them will be low.
Otherwise, to your enemies, they seem exactly the same as normal units.
In particular, for you they provide sight, so they can be used for scouting or spotting.