Bennett offered him $55.5 million over 4 years, and gave him as long as he could to agree to the deal. Harden called his bluff on the hour ultimatum, and he got his ass shipped out.
As for his quality--I think his issue will be more of a personality and role thing in Houston. In OKC, he came off the bench with no pressure (and usually had a lead to either consolidate or expand). Instead of playing with Collison, Durant, and YNB, his best teammates are Omer Asik and Jeremy Lin.
Even in college he shied away from being "the man" in big moments, and the sixth man role was perfect for him. He still has virtually no mid-range game, which will be a big problem when defenses are keying their entire defense around stopping him.
I'm still bummed he's gone, but I'm glad we got so much in return for him. Presti can do a lot with those kind of assets.
And I also agree that Maynor won't be on the squad next year. He'll be a RFA, and Reggie Jackson has apparently been progressing well.