By 1963-64, when the final season of Twilight Zone was being filmed/aired, the show had already gone through a cancellation, a name change, a change in length and was going on it's 3rd president in office. So many wonderous ideas had made it onto the screen that the well understandably began to run dry. In the realm of science fiction, quite often a writer will begin to rely on cliches more so often after years in the genre.
Such is the case with season 5 of Twilight Zone. Shows of this season lacked the precise structuring and engaging story lines of earlier seasons. Episodes such as 'Uncle Simon', 'Ring-A-Ding Girl' and 'Ninety Years Without Slumbering' simply lacked a feeling of involvement, and others such as 'The Bewitching Pool' were just poorly made, going so far as to intercut an overdubbed adult voice for that of a child character. Yet with all these complaints, I would be the first to say that with a show like Twilight Zone, I find myself returning to the clunkers with blissful abandonment, as there is a certain charm to even the worst that this series has to offer. And there are certainly plenty of clunkers available here for those viewers so inclined to induldge.
But in a strictly objective sense, season 5 was arguably the worst of the series, with season 4 the only other contendor (although the different length format makes the two almost impossible to compare). However, the winners of this season unquestionably stand with the best of any other, amongst them being 'The Masks', 'Living Doll', 'Garrity and the Graves', 'Steel', 'In Praise of Pip', 'Last Night of a Jockey' and probably the most well known of the season, 'Nightmare at 20,000 Feet'.
The probable cause of the downfall of this season (other than Serling just becoming drained and disinterested), as mentioned in Mark Scott Zicree's companion book, has to do with the fact that the final 10 to 15 shows of the season were written by authors who had never previously done work on the show and who probably had no deeper understanding of the mechanics inherant therein. Twilight Zone always had certain thematic elements that were steadfast, involving evil getting it's 'just dessert' alla 'Death's Head Revisited', and depicting human tradegy as devastating, alla 'Time Enough At Last'. However, you will notice that shows in the 5th season written by these lesser authors thematically violated the time-tested zone structure, although again I find this to be curiously interesting from a fan's point of view.
I suppose the bottom line is that, regardless of how poor many of the shows from season 5 were, they are still better than 99% of the trash on TV today, and any fan of the series should own this one. Flaws aside, the gems included more than make up for any shortcomings.