I’d recommend posting the same review twice, once with a thumb up, and once with it down…
As for the original question: I very much hope I will not sink that many hours into a game only to find out, ultimately, that I really didn’t like it all that much. But I can see how it could happen, how you could slowly but surely find out the game isn’t as good as it appeared to be. In my personal experience, after all those hours, it would probably still result in a (slightly) positive review, but that’s just me…