If you buy this with ODST it's good but the original campaign sucks. If you get some friends to play with you in custom games, you'll have lots of fun playing on maps people created using forge. With forge you can create anything you want and share it with the world. You can download maps too. Some of my favorites are Lava Pit, Sky Castle, and Racetrax. I give Halo 3 4/5 Stars.
Good story in it. Nice gameplay will keep you occupied for hours. Online gameplay could have been better especially the spawning in the right places would be a nice update to get. Some ups and downs on the maps, but it still holds some nice online gaming from time to time.
Okay know if you love shooters man look no further this game is very well rounded if you like shooters especially if your a die hard fan never underestimate Halo and Bungie on the look out for Halo Reach as well.
I have played the campaign many times myself and cooperatively. People in multiplayer matches are two good maybe because American players may have an advantage so that European connections are lagging all the time.