I have to say that this is by a VERY long way, the worst episode of "The Prisoner". It suffers greatly from the fact that Patrick McGoohan is absent for almost the entire episode. The fact that he was the one and only cast member made his absence all the more notable. I missed his brilliant sense of humour, hilarious one-liners (like "Oh, I'll just go to pieces" in "The Girl Who Was Death" and the aura of coolness he gaves off every time he's on-screen.
I usually love body swapping episodes in any series but the main attraction of them for me was seeing what the bad guy is doing in the good guy's body because it gives the actor a chance to flex his acting muscles, not what the good guy is doing in the bad guy's. All the Colonel does in Number Six's body is lie semi-conscious on a bed wearing stupid looking goggles!
I thought Nigel Stock was a poor substitute for McGoohan as well. He was a good actor but I never really bought that this was the same stubborn and extremely intelligent and resourceful man that I'd watched in the last twelve episodes stuck in another man's body. In fact, the only time he seemed to be acting or even talking like himself was at the very end when his mind was put back into his own body.
One other thing that bothered me was that Seltzman believed that he was who he claimed to be after just comparing two samples of his handwriting, one written while in his own body and the other written while he was in the Colonel's. It could easily just have been forged. A man capable of inventing a machine capable of swapping two peoples' bodies should have realised that. The final twist, however, was brilliant and I did not see that coming at all.
All in all, the low point of "The Prisoner" but every series has to have one fairly poor episode.