The opening with the ice planet and the red goo virus will always make this a classic Trek episode for me. In an interview the writer said he wanted an episode were everyone was drunk but had full co-ordination. Watching now with an adults perception I can see where he was going with this concept! I think after they get the cure though, those crewmen must get one hell of a hangover!
Interestingly at the end as they have regressed 3 days there must be another Enterprise zooming around the galaxy until they catch up to their current time line!