We've got quite a few Access customers still on 2003. Yesterday one asked if they should upgrade to 2007. The answer, of course, was "no." Naturally the conversation was directed to whether to upgrade to 2010 instead. You can find highlights for Access 2010 here. In the end, it's probably time for most users to upgrade to Office 2010 if you have 2003. The fact is 2003 was a legacy of Windows 95-era desktop apps, while 20007 and 2010 are more representative of Windows Vista/7 user experience design patterns and security. Not only will you find it easier (after some adjustment of course) to work in Office 2010, but security is also a key element to consider.
Access 2003 introduced a minimal security prompt, whereas the Office 2007/2010 Trust Center treats security in Office with the consideration it deserves. Office 2010 includes file-level trust, something definitely missing in 2007.
Lastly, if you are considering upgrading to Office 2010, you can reconsider whether all users require the full version of Access, or whether you can save money with the free Access runtime edition.