I think Samba is an excellent replacement for windows server for simple filesharing, is usually easy to setup and some distros even drop in powerful GUI configuration tools.
I have used samba in a small office (around 10-15 office workers), with a few shared folders (around 5 GB of documents), at first the company didn't trust our use of Linux, they had a windows 2000 server which was badly managed (and filled with virus/malware and being used as spam relay), we gave them a 1 month complete guarantee that the system will keep up without any problems or we give their money back and install w2k server back.
They are quite happy now as once of properly configured you don't need to mess with it, we even added virus scanning (via clamav and hourly cron, samba clamav plugin taked a noticeable performance hit and was not straightforward to configure) and reporting via email (plus the email system running on the same server).