I've got the US Region 1 DVD box set of Series 1 and 2 (Series 3 should be arriving real soon).
It gives you the option to watch each episode one by one which shows them as aired. Alternatively, you can play through the lot in one go and by doing it this way you get extra scenes, sentences, looks .. very small and subtle and you'd only notice them if you were a Friends nut (I figure I'm in the right place here!).
Anyway, as a bonus on Series 2 you get the whole video of Smelly Cat, not just the bits they show on the episode, but a whole proper video.
And in the episode where they all sing it with Phoebe in Central Perk (can't think of the Episode name right now), they all sing it like idiots falling about laughing which wasn't shown in the original episode. Quite lovely.