Short version: It's good, buy it if you're crazy enough to not have bought it already.
Not-so-short (but still pretty short) version: Halo 3 really delivers on all the promises it made. The single player concludes the story, but ends with a possiblity that something else could be coming. Ending was pretty good, could have been better if they went into a little more detail, but it was fine.
Multiplayer is everything it should be. Feels virtually identical to the beta, just with more features. Saved Films and File Share are my new favorite hobbies, they're really addictive and it's fun to see the crazy stuffsome people have done. Forge isn't really my cup of tea, Irarely (read:never)play custom games, but the tool is surprisingly easy to use and the possibilities are virtually endless as far as custom maps go.
One of my few dislikes about the game are the matchmaking playlists, or rather the gametypes they play. I'm a huge fan of Shotty-Snipers, as well as some of the other types too, but there should be a little more variety to the types matchmaking puts you in. Also the map selection doesn't feel as strong as Halo 2s was, but there are still some very enjoyable maps present.
All in all it's a phenomenal game that backs up everything it ever said it was going to be.