Well, the truth is...
The number one reason to upgrade is that MS will eventually stop supporting the older versions. Now, that won't matter to some, and if not, stay with what works for you.
The truth is, I like Access 2010 -- I didn't like Office 2007 and so it seems a bit odd to know say I like Office 2010, but it seems to me like many of the 2007 irritants have been fixed. In addition, now that I have my head wrapped around the ribbon interface, I can find most things -- it's just like the menu feature, once you understand the hierarchy, it makes sense and you know where to go looking for things (mostly).
Can't speak for development though -- developers hate the new accdb format. Most are
