So good for the first 90 minutes that I'll forgive the last thirty where things just seem forced. An avalanche out of nowhere (???) and the extra army that seems to only really be there for added spectacle.
The reason this works is that the relationships and attitudes of the apes is so well done, that you don't even think about it as a special effect. For a while, this has an epic feeling like a classic Bridge on the River Kwai or Lawrence of Arabia feel. For a movie about monkeys, that seems to be a little over the top, but it really does work for quite a while.