Here there be spoilers
I recommend doing Operation Anchorage as early as you reasonably can. I just redid it, and realized that one of the rewards for completion of that pack is power armor training -- so if you do it early enough, you'll be able to wear power armor way before you normally would in the game. (Unless you were fast-tracking the storyline missions. But who does that? :)
For the others, there are definite advantages to doing some early (Mothership Zeta comes to mind, as some of the new items from it are really powerful), but as C.McAtackney said, the enemies can prove more challenging in some areas (notably Point Lookout).
On the plus side, now that you're playing with all the add-ons, you'll be able to go up to Level 30 ... And that's regardless of whether or not you even make it to the Broken Steel missions. :) The way I'm currently replaying, I'm liable to hit level 30 before I find Dad ... :)