This afternoon I got a chance to take my boy to see the Clone Wars. I'd heard it wasn't very good, but I really enjoyed it. My son LOVED it!!!
It's timeline is between the Clone Wars cartoon on Cartoon Network and Episodes III.
Also, some of the characters in this movie are from the Cartoon Network show. So I won't say you'll be lost without seeing it, but it will help.
Anyway, if you like Star Wars, there is a good chance you'll at least enjoy the movie.