I barely started Point Lookout and I think it is a lot of fun.
Lots of fan speculation going on, but there are 250 achievement points that Bethesda has left for the Microsoft achievements. This is the same amount of achievements that the expansion Elder Scrolls IV: Knights of the Nine expansion! This means we'll could very well get a Commonwealth expansion for Fallout 3! Truly awesome if it is true! (There have been plenty of hints to the place in the regular game, plus The Pitt, Broken Steel, and now Point Lookout!
http://fallout.wikia.com/wiki/Commonwealth