I hate a-la-cart multiclassing ("I'll take a little fighter here... a little rogue there... mm... a little more fighter, I guess... and maybe a little bard on the side"). I hate it especially because it tempts people away from their character concept for the sake of optimization. They start out as a pure-class Warlock, but then they see that they'd get a nice little DPR upgrade if they took a few levels of Sorcerer, and suddenly they're considering compromising their character's principles just for a little more DPR. Awful, awful, awful.
Multiclassing worked so much better in AD&D. If you were a Fighter/Mage, you were a Fighter/Mage for life, starting at level 1. I wish they'd go back to that system.