I agree that the Season 5/6 era was a bit bizarre. There were two or three really good ones, so I think I will still buy the set. And except for "The Conspirators," I think season 7 is fantastic! But I don't know if I will buy any of the 80s/90s episodes if they make it to DVD.
Season 5 is worth having IMO simply for great episodes like "Now you see him...", "Forgotten Lady" and "Identity Crisis". I consider the entire season pretty good. It was Season 6 where the drop-off was noticeable.