This show is my earliest TV memory---my father loved this show and we watched it a lot together when I was very small. I recently discovered Seasons 1 and 2 by chance at my local Fred Meyer, picked them up and enjoyed them immensely. I was reminded again of how funny Williams really is.
Robin is at his manic best, and it's obvious that he often forsakes the script and happily heads off into improv la-la-land. Some of it is of course dated now, but it is still very funny. People sometimes disparage Pam Dawber, but she did exactly what she was supposed to do---be the straight man (or woman). Williams is SO manic that he needed an EXTREMELY normal, average straight man to contrast with, and that's exactly what she is. She's cute and perky, the quintessential girl next door. When she explains basic human nature to a confused Mork (which she does constantly) she seems totally believable, like having an older sister explain something about people you didn't understand before.
Watching it as an adult, I did notice a few things I didn't realize as a child: Pam Dawber spends many scenes trying desperately to not laugh and break character at William's improvisations. You can see it in her face; to me, it makes it even more entertaining. Also, the live audience contributed a lot to the general air of cheerful hilarity on the show. When Mork or Exidor show up for the first time in each episode, the audience literally screams in delight.
All in all, watching the first two seasons again I was greatly entertained, laughing throughout, and it brought back great childhood memories. If you're into checking out past decades of pop culture, you need to see this show.