They work darn near seemlessly. This simple scalability, in my opinion, is one of FMP's major advantages when compared to similar products. (Take a look at what's required, when you need to scale up an MS Access system to SQL Server or a similar product sometime.)
There are a few details to keep in mind that you can allow for as you design your single user system. Global fields behave differently and it helps to read up on record locking. Record locking is how the server keeps two users from modifying the same record at the same time.
For more on network hosting your DB. See this knowledgebase article: KB 2984