I never get enough of this show. I can see the same show 3 times in a day and it's still funny even though I know what is going to happen.
I liked the first Darren more than the second one. Dick York just suited the role better. Dick Seargent just didn't seem to have the chemistry with Elizabeth Montgomery that Dick York had. Had Dick York's health been better, he would have been in the show for the long term, but he was replaced because his health (back problems I think), caused him to miss too many taping days.
I also liked the original Gladys Kravitz better, however, her her health made it impossible for her to continue in the role too.