I missed my dose of Effexor last night and I am paying for it now. I get severe nausea and irritability bad enough for my husband to drop everything and run to a pharmacy and a pharmacy to give me an emergency dose, if that's what's needed (I forget to call my refills in). I've never had any other withdrawal symptoms though.

Edit: Oh wait, the night I do not take my meds, I usually have nightmares and night sweats