I just exchanged Lydia for Erandur, and a few issues came up right away:

Y'all may find this humorous - if you give Erandur some mage robes, and some heavy armor, not
including the armor suit itself, he will choose to put on the other heavy armor items (gauntlets, shield, etc.), and go bare-chested - check it out:
http://img853.imageshack.us/img853/189/erandurbuff.jpg


Check out those abs!
On a more serious note, this is my first magic-using follower, and it raises some questions:

• will he use potions of restore-magicka and so forth?
  
• will he use staffs? If so, will he somehow recharge them, or do they never run out, or does he use 'em until they run dry?
  
• is there a way to get him to learn new spells? Can I give him new spellbooks?
  
• It seems he 'should' be wearing mage robes - is there a way (other than never giving him heavy armor) to get him to overcome his excessive fondness for such gear? Or does he know something I don't about how he should best be equipped?

Thanks in advance for any help, may no arrows ever find your knees,