Really good, just not living up to all the hype.
I'll break it down now Campaign: Very challenging, and fun, but just lacks a sense of purpose in combat. Allmost like you're doing nothing but shooting your way through and no tacticalness.
Xbox live: Awsome online. But in team gametypes, there's not very much "team combat" to it. It feels more like a free for all, but with people you cant shoot.
The ranking systems are great and give you a reason to keep playing.
Extras:
The file share, screenshot tool, and film clips tool are extremely cool. A really great extra. Forge is a good extra too, but needs to be more in-depth. The armor upgrades are a major reason to unlock Acheivments, even if you aren't a gamerscore whore. Totally worth it.
Acheivements:
The achievements are pretty screwed up. Almost 90% of all gamerscore in this game is campaign achievments. If you beat the game on ledgendary, all you need to do is get a couple of online achievements and you'll have them all.
The final statement:
The Halo series is a great series, and I admit, Gears of War has stolen a bit of it's thunder and Halo 3 has put them up to speed. Though I must hand it to Gears for a better overall game.