The Cage is my favorite episode of Star Trek. As you say, when The Menagerie was aired, no one had seen The Cage, so it was brand new. When I first saw The Managerie as a kid, I didn't realize they were reusing footage from the first pilot. I thought they filmed those parts for the episode. And, I didn't think it was boring. YMMV.Without a doubt the worst episodes are "The Menagerie" part 1 and 2. It's like episode "shades of grey" from the next generation which many consider tng's worst episode. Menagerie is basically two episodes of the crew slowly watching episode "the cage". It is quite boring. I realize back then "the cage" had not been released yet but we're not there anymore. I'm surprised this episode was not mentioned somewhere in the last 11 pages of posts. Just watch "the cage" and "Menagerie" part 1&2 right after and you'll see what I mean. Worst episode, hands down.
While I prefer The Cage to The Menagerie, I don't find the latter to be boring. Actually, some of the new parts are fairly good.
Mr Awe