Star Trek is as significant a cultural phenomenon as any - Elvis Presley, the Lunar Module, JFK assassination, 9/11...
I don't mean it is equivalent in importance. But that the nation and culture are so inextricably steeped in it, that without it, the course of a nation and the world could be significantly different than it is. American and even world history would be significantly different than it is without Star Trek.
And for all Shatner's obvious shortcomings, he is one of a handful of people who shaped that phenomenon. It may not even exist with the significance it does without him.
It's interesting to me to watch the old series and notice how unabashedly American it was. Kirk was the embodiment of the robust and unapologetic American frontiersman, and the Enterprise was aptly named as the crew brought American values to the galaxy. The later permutations were tinged with collectivism, even while shining a completely negative spotlight on the end-result with the "Borg" episodes.
I can appreciate people not liking Star Trek. It's not for everybody. I don't like Jimi Hendrix. But I certainly appreciate the cultural impact of Jimi Hendrix and his influence on practitioners of his craft, and I think that anyone observing Star Trek can probably see that same thing - maybe not their cup-o-tea, but an important and impactful piece of Americana that is probably a net positive.
Re; spinoffs... "TNG" was a good reboot in IMHO. "DS9" took a season or two, but it became the best of the spinoffs IMHO. "Voyager" sucked eggs. "Enterprise" - never watched it enough to form an opinion.
I liked all the films to one degree or another. I cried when Kirk died in the "handofff to TNG" film. I really, really liked the reboot that came out a couple years ago a lot. I liked the casting, and as a diehard fan of the original, the changes they made to the characters didn't bother me. In fact, I thought they took character traits from the original portrayals and expanded/magnified them, rather than just making old characters act out of character.