The first Halo campaign was incredible. Halo 2 was pretty good. Then Halo 3 came out and blew my mind. ODST and Reach are the only others that have come out. ODST started out kinda slow, but it got really good once you really got into it.
I played the multiplayer beta for Reach and watched all of the trailers. It loos awesome!
I have the ODST, but I haven't played it much, too lazy atm, haha. In what time can you finish ODST?
And Also, my friend has the 3rd Halo, which I could borrow - should I finish the 3rd first and then ODST or it doesn't matter much?