I love this episode. Monica and Chandler start their life-changing relationship and Ross and Rachel get back together, sort of.
One of the best moments of the episode is when Rachel walks in, hoping to stop Ross marrying Emily, but she sees them kissing, and she decides to congratulate him instead.
I think the appearences by Tom Conti and Jeniffer Saunders are great too. I think they're truly terrific actors, and they really did make this episode a lot funnier.
Another great scene is where nobody laughs at Chandler's toast (I'm guessing this was to make fun of the fact that English people have such a different sense of humour to Americans).
Anyways, I think this episode is underappreciated (even if it is currently the 5th highest ranked Friends episode on this site), because it is fabulous. It has a lot of similarities with 'The Last One' too, which of course, is only a good thing.