Psionics are pretty good, almost all my parties so far have one. In Wiz8 the most useful spells are the ones that incapacitate enemies and that's where the Psionic excels. Nothing compares to slapping an Insanity spell to a group of critters and watch them beat each other up while you rest your feet
A Bishop can learn those spells but a much slower pace, but it is a good substitute. The only other choice would be the Monk and that will be even slower because most of its level up points are better used for pumping up the combat skills, so you won't be able to focus on the magic side until much later in the game.
I think the trick to develop a good Psionic is to specialize in a couple of the magic realms (Mental and Fire) to pump as much as possible and then work on Divine spells later on, which include the Eye for an Eye spell that Vaprakgruumsh mentioned .