This is one of my favorite episodes ever. You can really tell that Courteney Cox enjoys playing fat Monica and the rest of the cast seemed to get a kick out of playing a somewhat different character.
I also really like that in the end, they all end up more or less where they are in reality. Ross still realizes Carol is a lesbian. Rachel catches Barry cheating on her. Chandler and Monica end up together. Phoebe ends up singing at Central Perk. And Joey has all of his fun stuff (the dog, raining window, etc.) Ultimately, we see that even if they haven't all followed their dreams, they've ended up in more or less the same place.
Great episode, nice change of pace from other episodes.