In my book, Ad Hominem would not be to point out that a fresh new one-day-old user isn't credible when discussing user experience on a site. After all user experience can't credibly be discussed by people who are not users. It misses the point. Even if the user in question happened to be a user 4 years ago.
OTOH, Ad Hominem would be calling other members "Old Wine", "old whine" etc. in order to discredit their legitimate viewpoints. Also referring to current status as "their beloved blanky or teddy bear", "crusty old interface", "lava lamps and shag carpet", comparing it to a "crap hole of a restaurant", and referring to other members viewpoints as "glib clichés" is certainly derogatory.