On the one hand, Facebook is a hugely popular website that made about $1 billion in profits (not revenue, but profits) last year. This implies that its stock should be worth a lot of money going forward. However, it really isn't all that clear that the company can continue to make huge amounts of money. They only get about $1 per user. Many other internet companies have seemed like they'd make a lot of money, only to fail to figure out how to turn users into dollars.
Personally, I would not buy into the IPO. It is hard for me to believe that Facebook will become more popular and more lucrative than it is now. It's a great business now, but will it be able to grow enough to give me a good return? I'm not confident.