I've just found 7 of them (which is exactly as much as I can take atm) in the chapel on Scarlet Wind, and am finding them extremely useful, what with their area attack that curses enemies, magic shackles that will render those pesky assassins useless for a couple of rounds and, most importantly, the 3x raise dead that's a marvellous distraction both for blocking the way and for sucking up retaliations.
Believe it or not, I've never used necromancers before, and that includes The Legend. *gasp*
What is your experience with this unit, and are there any units you've sort of discriminated against but was pleasantly surprised once you gave them a try?