I voted for not completely and I'll try to explain why without doing the spoiler thing, so mods get ready to possibly edit. =)
First off, what we know, is that its about 20 years prior to A New Hope (ANH). Second, there is a lot in ANH that actually could be answered as questions to what happened to? For instance, (and i'm borrowing from a post on theforce.net), the supposed Jedi purge, Obi Wan tells Luke of Vader helping to hunt down and destroy the Jedis. Do we need to see this? Not really because we are told about it in ANH. Just like we won't see a lot of the Clone Wars because it was starting in AOTC and ends in EP3. We know Obi Wan served in it, based on Leia's statement in the recording from R2 in ANH. The key point here is to remember that its "Episode 3" while it may be the final one for Lucas, its the middle of the story. You just need to look at them in order. I'm sure there are some things that Lucas should answer in Ep3 like the disappearing when a Jedi dies, but again, most questions can be answered by watching ANH.