We are a desktop app where 90% of our users hit a local data set on their laptop while out in a vehicle or remote location. The other 10% replicate with a master database via our own replication process. For our standalone users Sql Express was the answer. Not only is it free ( part of our install along with .Net framework ) but if you ever want to upgrade to a server it is the same product basically. We design for everyone to hit the local and if they want a server the connection string is the only change required. Replication is handled by the DBA's as far as we are concerned.

Another factor in us changing was client IT departments insistence on two things: change from a fox database now and in a few years no fox app layer well before MS support stops. All I will say is .Net is the future for 3/4 the market as it stands today so keep that in mind.