Designed to keep you from forgetting when it's somebody's special day, you can set up reminders that can be sent to both you and your friends. To do this, the service has employed a system that pesters your friends to add themselves to your personal database. In the process, the hope is that they'll join the system too.
Right now you can do this via e-mail, IM, Twitter, and Facebook. In all cases, it sends out a quick mm/dd/yyyy request to your contacts, along with the option for them to hide the year they were born. There's also the incentive for them to sign up, since when they're done putting in their birthday, it tells them when yours is, and gives them the option to squirrel it away by signing up themselves. Once they've done this, it's added to your database where you can filter them down into special groups like friends, family, college buddies, etc.
My one major beef with this service is that if you're trying to get birthday information from your friends on Facebook, it's likely already there. There are also a handful of apps within Facebook that offer this same functionality. Also, since November, Facebook has had its own weekly reminder service that you can sign up for that tells you whose birthdays are coming up. For that reason, MyPunchbowl's service ends up being a better solution for Twitter, although I'll be honest here--I don't care about the people I'm following on Twitter as much as I do about the friends and family I'm connected to on Facebook.
Where the system holds real value is that it's deeply tied into MyPunchbowl's other services. For instance, if it's someone you were never intending to get a gift or a real card for, you can simply use MyPunchbowl's eCard service and send it to them using the information you've already gathered. By contrast, you can also turn around and begin planning a party right away using the built-in tools. For that, there's some real value, which is what makes it worthwhile if you're already using the service to do the party planning. If not, you may be better off using a Facebook app like Birthday Exporter to take that birthday data to a calendar service you're already using.