Cleverly-played wizards are frighteningly effective. Hjorimir, who posts on these boards, plays wizard NPCs very well--I recall an encounter wherein a lich single-handedly picked apart our entire group of 7 PCs + 2 cohorts with a few well-placed spells. He killed two of the PCs and took their stuff, then he left. The party was 12th-13th level at the time, and the lich was an appropriate CR challenge.
Here's how it went: he opened the fight with
Otiluke's resilient sphere on the juvenile gold dragon cohort, followed by a
wall of force that divided the party. Then he dropped a
wall of iron across a doorway that further separated the group, leaving the fighter and cleric on the side with him. Next, he trapped the fighter and cleric in a
solid fog, and then finished them off with an empowered
cone of cold. Once they dropped, he looted their bodies (!), and then left via
dimension door before the rest of the group could manage to get through the
wall of iron and the
solid fog. Why did we give him so many rounds to work his magic, you ask? Let's just say that we spent most of that time attacking the bastard's
programmed image.
Wizards, properly played, are straight evil.