Halo is a bit of a strange series. Every game in the series gets better consistently the more you play it. Some other games do that, to some degree, but usually most things get really dull by the third or fourth time you play it. Not so with either of the Halo games. There's ALWAYS stuff you haven't seen, haven't heard and haven't experienced. I found an awesome sniper spot and an exciting alternate way of taking down a troublesome enemy on the fourth level. That would be the fifth time I played said level. I'm going back to it tomorrow or the day after that. And with tens of thousands of randomly selected combat dialog from all types of enemies and allies, you will hear something you haven't heard before almost every time you play it. Also, the better you get, the more you'll want to experiment with weapon setups and tactics, and the more you experiment, the more hidden goodies you find spread around every level in the game, which in turn makes you even more willing to explore. And that makes you wanna play it again. And again, eventually.
Then you play it with friends, and it's a completely different game, entirely... Me and a friend were playing two or three levels of the first game almost every day, for almost a year. I swear. It like was an after-school tradition for us. Same levels, same time, every day for many many months, and it NEVER got boring. Halo 2 didn't have quite the same charm, but it DID have more eggs and secrets to find even when playing alone.
Halo 3 is better than Halo 2 was. Which probably means I'll be playing it for many more years, even. Especially with the sexcellent multiplayer. I've never had the chance to play Halo online until now (not counting the H3 beta), and it's extremely exciting.
In short: If you play Halo 3, it'll seem avarage. If you play it twice, it'll seem like you'll be bored of it if you play it one more time, but you'll start to understand the really complex story. If you DO play it one more time, and try a higher difficulty, you'll start to wonder if the amazement never ends. The first two games were exactly the same in that regard. Hell, I last played Halo 1 on co-op the weekend before Halo 3 was released. The first half of it was STILL incredibly fun, although it felt slighty slow and, in my case, routine. Probably because I've done it a billion times before.