my main is a N/Me, and (when i actually pay attention and try playing while fully awake, not over-tired and half asleep :P ) I find her quite effective. She specializes in Curses, Illusion and Domination magic. She began her adventures by first cursing and hexing and bleeding her enemies to death, then raising them as undead minions, but the minion animation took too much time and effort to keep herself surrounded by an undead army at all times. (plus, more or less helpless while fighting things that cannot later be raised as undead...such as rocks and ice..things!) So she got rid of the minions, and focused on the diabolical side which allows her to crowd-control, hex, curse, dominate, crack some whips, debuff, disable, send plagues, and watch the mobs kill themselves for her.
in PvP i've heard rumours that a combination of Necromancer and Mesmer can release a virtual plague and spread disease through the ranks of the enemies... but, I don't really PvP, though may check it out eventually.
P.S. Mesmer/anything, or anything/Mesmer, is more or less guaranteed to be uber, imho. with the right skills, and attention span of course.