This episode was interesting. On one hand it was a bit boring that they were re-using old lines, plotlines and jokes. On the other hand it was the best flashback episode I've ever seen.
I mean, if you're going to waste an episode on old material then why not do it with some style? Comparing it to other shows, say "Friends", it's done much better here. On "Friends" the flashback episodes really felt like "we couldn't bother coming up with much new material, here's some old stuff". In this episode it felt more like a nice trip down memory lane. For one thing there was a lot of new material, for another it had to have taken a lot of effort to weave in old material into the episode, and for another it was done so that it didn't feel old. Those who watch the show, but don't watch the episodes over and over, probably thought a couple of old lines were new. It felt kind of fresh. And most importantly, it felt like they were trying to give us a nice look back at some of the things that have happened, rather than trying to take a short-cut and produce a new episode with little new material.
I'm not a big fan of flashback episodes, but this is the best one I've seen.