I liked it a lot. It's not without flaws - there are some timeline shifts that are a bit confusing at first - but Achebe does a great job of creating his characters, warts and all; Okonkwo isn't the most unproblematic hero, but he's a great character.
Given its reputation, I was half-expecting this to be "we Africans were living in perfect harmony until the evil white man came and massacred us", but it's very far from that. It's not so much about saying one side is completely right and one is completely wrong, but more of describing the effects of the much-bandied-about phrase "culture clash". Several of the Christian missionaries in the book come across as genuinely kind - if ignorant - people. At the same time Achebe is determined to kill the myth of "savage Africa"; the many details of daily life in the tribe serve up an image of a society which has a lot of faults - violence, superstition, generally a hard life - yet is in its way a well-ordered society with laws, traditions, friendships... And what kills it isn't gunpowder but the loss of hope; the last chapter is one of the most brutal sucker-punches I've read, but it makes perfect sense. And yet there's the defiance of the title, appropriating Yeats' quote for the Africans, turning it back on its home country.