It's funny how in recent years, the character of James T. Kirk is no longer strictly defined by William Shatner - and I don't just mean Chris Pine's interpretation. Thanks to the efforts of Fandom Assembled, your Kirk could just as easily be Vic Mignogna or James Cawley too, depending on how you became a Trekkie.
It's hardly considered unusual anymore. Indiana Jones, Han Solo, Lara Croft, they're all getting re-cast with younger stars in new stories - and of course, the James Bond franchise has carried on this tradition for over half a century.
Maybe that has something to do with the power of modern myths. As the story gets passed down from generation to generation, some changes are inevitable to suit the changing times. The core of the myth, however, remains the same. Kirk: explorer, diplomat, defender. A born leader, ever conscientious of the lives entrusted to him. A man of action, the embodiment of the American self-image during the Kennedy years and his plan for space travel in specific.
People make fun of Shatner's acting on the show, but I think he's just a product of his time. He's not a Brandoesque Method man; he's more of a throwback to the old Hollywood, which was more about the force of one's screen persona. Shatner radiated that force, that magnetism, as Kirk, and if he came across as more... individualistic than most actors, well, there are worse cinematic crimes.
We know now that Shatner may have stepped on the toes of some of his co-stars. We also know Shatner himself felt threatened by Leonard Nimoy's popularity for a time. I don't think knowing these things detracts from the quality of his performances, however much we may wish everyone got along well. Shatner's got an ego, no doubt. At the same time, he's not afraid to have it punctured once in awhile.
Ultimately, if you're a Trekkie, you just have to accept Shatner for who he is, warts and all. He personified one of the greatest fictional characters of the 20th century for almost thirty years. I think he's entitled to strut a little.
Chris Pine's Kirk