No Doom3 is NOT worth getting for co-op alone. Its pretty fun co-op, but they cut out a LOT of the game. No puzzles, key cards, PDAs, access codes, storage lockers, etc. Maybe you didn't like those parts of the game and think it souns better with them gone. But whether you liked those parts or not, they added a LOT of length to the game. Without them in there, you will be finsihed co-op from the first level to the last boss in about 3.5 hours (thats how long it took a friend and I to play it).
It was fun, yeah for sure. But if your sole interest in purchasing the Xbox version is for the co-op play, rent it instead.