So, if what the folks in Redmond want you to do with the Express edition is to develop those "little" applications that just run on a desktop computer or a "small" server, how do you know if your project is small enough. With computer power and storage what it is these days, a small server could easily support several hundred users. A couple of small servers could scale well into the thousands. So, the question becomes, when do you need to get a paid version?
The best way to answer that question is to look at what you don't get with Express. Once you see a deal breaker, it is time to move up to the Standard or Enterprise editions.
Express only supports database mirroring as Witness, and does not support failover clustering at all. It also does not support online indexing, online restore, compression, hot add RAM or CPU, database tuning, data mining, or multi-dimensional analytics (whatever that is). More importantly, the Express edition does not provide the same level of support for integration with other data, like using XML source data or Oracle Publishing. Express also does not support Transact-SQL or MDX, so if that is how you are used to doing your thing, you'll need a different edition.
If you haven't seen a hot button, then give it a try. After all, it's free and I've heard good things about the Express edition of SQL Server Manager Studio. (We'll look at that in an upcoming article.)
Here is the download page for Microsoft SQL Server 2008 Express.