Off season doesn't really exist anymore. Value season is the best way to go, because the resorts have the best prices and the crowds are less. IMO, the last couple of years we have went during value season we have still faced crowds. I think the really cheap airfares and the promotions such as free dining have changed the crowds a lot. When we first started going to WDW, rt was always $400 or so, sometimes more. Now, we often find fares $250 or under, so we go more often. Obviously we are not the only ones!
The official WDW website is a great planning tool for knowing ahead of time which attractions may be down for maintenance. When we go during value season, usually something is down, but it is usually only one attraction.
Definately go during value season! We've made the mistake of going during peak season, and it is miserable for everyone.