[Merkin]

My current party has only a psionic for magic, no priest or alchemist. As other people have pointed out, they are not as "powerful" as a mage, in mid-game nothing kicks butt like fireballs. But insanity is perhaps my favorite spell of the game, I just love seeing the monsters do my work for me. And the combination of slow on the enemy, haste on your party will have your fighters bowing down and singing your praises. And they do have some good damage spells as well. And if you are going to have a single magic user doing both offensive and defensive magic, I think they beat a bishop all to hell.

One tip. There are far fewer "found" spellbooks for a psionic, so really study the spells ahead of time, and plan carefully how to allocate points and spells as you level up.