I have used both versions; AutoCAD 2002 as a student for my CAD classes and drafting/designing projects, and now I am using AutoCAD 2007 for reviewing, editing and creating drawings and sketches for my current employer.
I am not a draftsman (I'm an engineer) so I must admit that I am not familiar with all of the advanced tools included with the program. However I can tell you that the basics did not change from the 2002 version to the 2007 version.
There is a slightly updated user interface - in 2007 a command box appears next to your cursor when you are typing commands, it helps a bit, because you don't need to keep looking to the bottom of the screen to check your commands for typos. The 2007 version seems to be a bit more customizable - toolbars still can float, minimize etc but it all seems a bit more intuitive. (That is personal opinion though.)
All in all I have not noticed any major changes. If you are well versed in 2002 you should have no problem using 2007. I personally would not recommend upgrading your software (if it is for personal use.) The only TRUE benefit I have found to having the latest version is that AutoCAD 2007 can open a file created by any version of AutoCAD, but some of the older versions cannot open drawings created in AutoCAD 2007. The work around is to have the person creating files in 2007 to save the file they are working on as an earlier file type [such as AutoCAD 2000/LT 2000 Drawing (*.dwg) ] - an option in the "save as" window.
I hope this helps you!
EDIT: Sounds like the guy below knows a lot more about the program than I do. If you are an advanced used I'd suggest you listen to him not me. For the basic user I maintain that the basics are still about the same.