It's a case of "you get what you paid for". As far as I know - you get essentially NO support other than what is in the KB and BOL. I don't even know if they will let you open MS issues on SQL Express (the 250$ per incident thing).
As far as limitations, there are quite a few. It's easiest to see by looking at the "Compare SQL Editions" link from the SQL Server home page on MSDN.
Unless you're planning on running very minimal SQL stuff - you may find Express to be "too light".
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Your lack of planning does not constitute an emergency on my part...unless you're my manager...or a director and above...or a really loud-spoken end-user..All right - what was my emergency again?