A recent SQL Server Express evaluation came down to a number of key considerations.  Bear in mind that the network platform involved was Windows XP workstations running on Windows Server 2003. 

1. Can a SQL Server Express back-end serve more users than an equivalent MS Access platform?

2. Should we use SQL Server Express 2005 or 2008?

3. What are the installation requirements for SQL Server Express 2005.

I'm currently evaluating options for a database project.  A prototype has already been developed using MS Access to design user entry forms and reports. Ultimately, the application will have about 100 users - but initially it will be just a handful. Still, there is a worry that Access will fall over as these users arrive.  

It will be a number of months before a SQL Server infrastructure will be put in place - so we are evaluating MS SQL Server Express as an interim development & testing platform. 

I've been playing around with a developer preview of Google Wave which is entering the next phase of user rollout at the moment and generating a huge amount of interest in many quarters.