Oops. I'm a bit <img src="/ubbthreads/images/graemlins/sleepey.gif" alt="" />
Aura of Guarding is useful as an extra bit of protection. I maxed this with purchased spell books after getting to Ars Magicana and Verdistis, and used it until about 2/3 through the game. Actually, I found Spell Shield more useful, especially against things that did poison damage (the damage would come off the shield, and your character would not jerk around with a greatly reduced offense/defense).
