Shatner sold the show as far as I'm concerned. He put more heart and gravitas into his performance in the pilot than any three actors combined. There's a damned good reason why he considered himself the star - because that's what he was. As much as Nimoy, Kelley, Doohan and the rest were vital parts of the overall mix, the show would not have been as popular without Shatner. He took the lead in nearly every story, and anyone who really takes the time to watch his work can see what a dedicated and detailed performer he is. He put in a lot of subtle shadings, things you only really appreciate upon repeat viewings. Even such over the top explosions like "I'm Captain Kirk!" in The Enemy Within are worthy of note. This wasn't some guy screaming at the top of his lungs - watch it again, over and over - the control of the voice, the pressure he put on his face, and how he reels it back in after the big scream. Unlike any "evil duplicate" story of the era, this was an animal in a man's body. This wasn't "Admiral Nelson brainwashed by Russians."
Even later episodes, like The Enterprise Incident: watch him as he slowly unhinges at the end of act one. He's tense, twitchy, and builds his anger into hysteria. Granted, he had his off days, particularly when a director wouldn't take the time to bring him down a notch. But most often than not, Shatner was the energy in the series, even when he was feuding with Nimoy or Doohan hated him, his likeable personality came through. The chemistry he had with the others was amazing. He also threw an amazing punch and in a role like Kirk, that is vital.
I always thought Kirk was the most interesting of the big two characters. Spock was - and is - an amazing character, but he works best when he's interacting with Kirk. Kirk, however, was interesting no matter who he was with. This is borne out when watching any of the earlier first season episodes when Nimoy was relegated to more of the background and Shatner got the main stories.
Personally, I don't care how he is in person, it's Kirk that matters, and as Kirk he's excelled and Chris Pine doesn't come close to having his presence.
Anyway, most people who "imitate" Shatner are actually imitating Kevin Pollack and the like. If Shatner were anything like they portray him, he'd never been able to make a living as an actor.
