I finished the Point Lookout DLC a few days ago and that's me done every quest (+ maximum achievement points; 1450/1450) and at level 30 in the game. The DLC seems to be getting progressively better each time... I just wish they'd up the level cap a bit more often. Regardless, I've enjoyed every minute of this game and it's a joy to explore.
Point Lookout is actually more fun than Broken Steel but it won't be such an "essential" purchase for many people because it doesn't raise the level cap. I'm not sure of the reasoning behind restricting the experience the player can gain in what is a purely single player game. What would it matter if some of us were at level 50+???