I just checked out Geni. If there is such a thing as a weekend genealogist then guess this site might be great. It seems to be more of a networking site than a genealogy site. Where I could see this being a very useful tool is for a group planning a large family reunion.
First you enter your name and email. Then you can add parents and go back as many generations as you'd like. Why would I want to type all of that info in? As far as I can tell the site does not support .ged files for easy transfer of information.
You can add your spouse, siblings and children easily following the arrows. You can add the email address of any person, which generates an invitation to them to join your tree. They can then add people and email addresses to the tree and you have your own little family network that can keep on growing. Your tree is private only those invited can view it.
Here's where I have a problem. The edit tabs allow you to add quite a lot of personal information, birth date, city you were born in. From the main listing you can get my mothers maiden name. I bet you can see where I'm going here. Identity theft isn't prevalent enough, lets give them the info rather than make them steal it!
I have no doubt hackers could have a field day with this site but they aren't my main concern.
I invite my sister, she invites her sister-in-law, who in turn invites her cousin who invites his wife's cousin. Very quickly there are people added to my tree that I do not know, yet they now have access to my personal information. You do not have to fill in your personal info. But let's say my sister sees I left that blank, she can use the edit feature to add it for me.
For right now I think I'll pass but I'll keep the site bookmarked for our next reunion.