It's a usability nightmare. No one knows what OpenID is, I only heard of Facebook Connect from you mentioning it.

I once tried to establish an OpenID account for stackoverflow, but it was too much of a hassle so I didn't bother registering. Users can just reuse names and passwords. It sucks from a security perspective, but unless you can make it transparent to the user, you're pretty much boned because no one will want to use it.

Because facebook may not be around next year. In fact, given the way things are going for them, I would not be surprised if your site outlived theirs.

When it's something as crucial as keeping track of your customers and users, don't "outsource" it unless there is a very, very compelling reason.

Facebook covers only a small percentage of people online.