warlock/mage is the best support class in the game, making it a must have in end game raids. There's another thread about it, read that for details on why.
Warlock/champion is the best damage dealing combination. Warlock/champion won't be able to keep up in raids, unless they are specifically built for magic damage (which is never), and requires a lot of tp and item set-skills to bring out its true potential, BUT it can still be devent if played correctly (when I say "decent" I mean that you won't get invited to raids as a dps b/c you aren't an amazing burst class). As said in post above, its really strong in siege war. You can dish out a ton of damage, move very fast, and get a lot of hp, making it a strong pvp class.