I have to say, Halo seemed very good to me at first, but slowly, I started to dislike it more and more after playing it for so long. I'll start with my good things first: The story is quite compelling, there are lots of weapons, and there is lots of good action.
My bad list: After the first THOUSAND Convenant I face, it gets old. And not being able to actually save at a check point and play from that checkpoint again just makes it that much more annoying. Personally, I don't have the patience to put up with playing a stage over again because I can't load from my last checkpoint next time I turn it on. And seeing as how I suck at multiplayer, I have nothing to say for that.
But don't get me wrong, Halo is a great series. If the stages were shorter and the save your game at a checkpoint option exsisted, I would be playing it.