Really? Well then you probably won't like Mafia much more... Same basic dynamics, you can't customize the character himself (just clothes from what I understand), and the cover system is very similar.
If you're a big fan of 30's-50's Americana you might enjoy it more, but somehow I suspect that isn't the case
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And what about the multiplayer in GTA4 did you find lame? I didn't play the deathmatch modes very often, but the race/gta race/cops crooks/the co-op mode whose name I forget were all MINT.
I was a big fan of causing havoc in the race modes if I fell too far behind. Most people quit if they get way behind early- I would drive a bus backwards around the course instead.