coolest build I've seen so far with a warlock was an elf warlock 10/dashing swordsman 10. Actually works out in an amazing and witty way. So far our group has had lots of fun with it. Especially since he got this familiar, I don't remember how he got it. Maybe he dipped a class or something since I don't see familiar as a Warlock's class set. But its a pig, he named him piggay and is commonly used in his puns in order to get the bonus to damage for Dashing Swordsman. It's also a large round of facepalms and deep sighs whenever piggay gets mentioned at the table as he usually spins off going:

"PIGGAY! Destroy them, we shall certainly.....fry their bacon hmmmmmm....*wheeze with each breath*"