He's not thrilled
Takei had quite a lot to say about it in a 2016 interview with The Hollywood Reporter, and in a post1 on his Facebook page shortly after that interview went live; highlights from both include:
In general, he's happy to see an openly gay character, and he's "flattered" that they made Sulu gay as an homage to him:
"I'm delighted that there's a gay character,"
Let me be clear: I am not disappointed that there is a gay character in Star Trek. On the contrary, as I made clear, I am delighted that the Star Trek franchise has addressed this issue, which is truly one of diversity. It is thrilling to know that future generations will not see LGBTs go wholly unrepresented in the Trek universe2.
I am flattered that the character of Sulu apparently was selected as an homage to me
He feels as though Gene Rodenberry's intentions for the characters aren't being respected:
"Unfortunately, it's a twisting of Gene's creation, to which he put in so much thought. I think it's really unfortunate."
"I said, 'This movie is going to be coming out on the 50th anniversary of Star Trek, the 50th anniversary of paying tribute to Gene Roddenberry, the man whose vision it was carried us through half a century. Honor him and create a new character.
On the specific question of Sulu being gay, when I was first approached with the concept, I responded that I hoped instead that Gene Roddenberry's original characters and their backgrounds would be respected. How exciting it would be instead if a new hero might be created, whose story could be fleshed out from scratch, rather than reinvented. To me, this would have been even more impactful.
He's also unhappy about the implication that Sulu is (or was) "in the closet", something he thinks humanity would have outgrown in the Star Trek universe:
"I told [John Cho, who plays Sulu in the film], 'Be imaginative and create a character who has a history of being gay, rather than Sulu, who had been straight all this time, suddenly being revealed as being closeted.'" (Takei had enough negative experiences inside the Hollywood closet, he says, and strongly feels a character who came of age in the 23rd century would never find his way inside one.)
Finally, he expressed his displeasure to Justin Lin, felt like he was being heard and that changes were going to be made, but they ultimately were not; he's understandably upset about this, too:
Honor [Rodenberry] and create a new character. I urged them. [Justin Lin, the director] left me feeling that that was going to happen," Takei says.
"I really tried to work with these people when at long last the issue of gay equality was going to be addressed," Takei says. "I thought after that conversation with Justin that was going to happen. Months later, when I got that email from Simon Pegg, I was kind of confused. He thinks I'm a great guy? Wonderful. But what was the point of that letter? I interpreted that as my words having been heard."
After having seen the film...
CNET reports on comments from Takei made at the 50th anniversary celebrations in Los Vegas; now that he's seen the film himself, Takei doesn't think it was a very significant or impactful scene:
"If you blinked, you missed it," said Takei. "There are others who are dealing with LGBT issues much more profoundly." He called it "a whisper of a scene."
1 For the sake of easy access, I've taken a screenshot of the post in question
2 As has been pointed out in the comments, and elsewhere on this site, it's not strictly true that this is the first time Star Trek has portrayed non-heteronormative sexualities, though I do believe it's the first depiction of male homosexuality