I think they are both over-rated and under-rated in their respective fields.
The first game was really trend-setting in it's time, and to me is still one of the funnest games (especially campaign) to play.
The second game seemed to me as sort of "rushed", as in I found that they were really trying to appeal to their audience a bit too hard. Still and epic game, and by far the best multiplayer experience of all 3.
Third one is just visually stunning and finishes off the trilogy with the covenant really well.
All in all I think the story was amazing, I can't stress that enough. There's so much symbolism (to me at least) in the whole plot, that I just think most people aren't intelectually aware (or explorative) enough to see and relate to real world events both past and present. That's what I was reffering to when I said the games were under-rated.
I find that they have sort of botched the multiplayer balance on their latest release, and it revolves too much around power weapons, and who can get to them first. Or for instance who can sit in one spot the longest with a shotgun and get cheap kills when someone walks around the corner. Still fun though, and as a mapper for computer games I find the forge mode brings a lot to the table in terms of your ability to keep your interest in the game and bring new experiences to the table (same for the game-type customisation).
All in all a good series, but sometimes I find myself wondering why they develloped certain aspects of the gameplay the way they are on the last release, but hey, what doesn't have it's minor flaws.