well out of solidarity, as we're the only two recent posters here with no replies! I'll give this a swing.
I have invested in individual stocks for about 10 years, but everything I've read here an elsewhere (Jack Bogel), or like jlcollinsnh's famous stock series
http://jlcollinsnh.com/stock-series/ comes down to forgetting individual stock picking, and instead indexing. Which I am transiting to myself. So you might be asking this IPO question in the wrong place. I mean the Square could be a great IPO, or a crappy one, but the larger question is does this type of investment fit inside your larger plan for a financial future? Do you plan on beating the market, making individual decisions like this, over the next 10-30 years or whatever you timetable is? Don't get me wrong, I loved picking stocks, I spent huge time on it, and I will miss it. But the math is very clear, in the long run you loose.