Patches can grant the following boons. To my knowledge, each boon has a equal chance of occurring with each level up:
+5% Attack Bonus
+4% Physical and Magical Resistance
He also inflicts one of these curses with each level up:
On long runs, you're usually hoping for the resistance boons. Mana Burn is the most consistently dangerous curse. Teleport and Grid Reveal can range anywhere from slightly helpful to game ending.