All DLC's for Fallout 3 should be similar to Point Lookout in that the character should be able to come and go. These DLC's where it gives you a specific quest that you must complete once you start are not nearly as fun and entertaining as those that allow to character to come and go. I imagine Mothership Zeta will be similar to Operation Anchorage in that once the quest is started the character cannot exit. And what's worse is that once it is over, there is no going back. I don't imagine that after the main quest in Mothership Zeta is complete that we can call up the aliens to abduct us again. When I think about the amount of time that I have spent on DLC's none of them are going to come close to Point Lookout because of the ability to leave and come back.