I saw his hatred developing into love from miles away (really, the manga description just hints you that much if you give the plot some proper thought) and time and again I was reassured of this ending. I think whoever got upset about this had a blank mind while reading, I think it was just that obvious.
Him "liking" Yoshino was more that he was confused about everything in his life; having your sole objective in life revealed as utterly meaningless can be mind numbing, and he's a young boy, so he lacks perspective and experience -- that he's young is also why he sways between probably not liking Aki and probably liking Yoshino. Young people often know jack about what they want or feel, especially in times of distress. Him going to his gramps to sort his head in peace and then finally making a stand is nice.
All in all, I think this is very well written, and the people who despised this ending either didn't give this story much thought to begin with or they just wanted their favorite girl to win. Lol at people going "b-but Yoshino is best girl", she's the sole reason behind that entire mess. She repented, made up with Aki and confessed (and got rejected). Any more than that would be forced for the sake of some "muh side girl should win" idiocy, and I'm glad that the author didn't head that way. That people somehow thought that the author didn't know what to do with the story just shows that people didn't read this for the story, but for some fleeting fulfillment of watching a story have an alternate ending of their liking, good sense be damned.