As much as I love the idea of intentional community, my family has btdt, and there are some definite downsides, even when they're started with the best of intentions. It would need to be a *really, really* loosely organized, absolutely voluntary, and egalitarian. But it's hard to hold a community together with that setup.
Community that rises more organically out of local friendships and family relationships, that's another story.
Anyhoo--we are fortunate to have parents who have helped create a little community. Their home has a revolving door, and adult children (along with grandchildren) are always welcome. When dh was oversees, my boys and I lived with my folks for 18 months. My little sister moved back home when she got pregnant, and they have helped her with raising that baby and going to nursing school. My other sister and her husband moved home a few months ago, and she just had a baby last week. They won't be there long, but it's great that she can be home and have the encouragement and help of my parents while she adjusts to motherhood, and possibly while her husband is in basic training for the Air Force. We are very fortunate to have the parents that we do, who *want* to be involved at that level, and enjoy it.