One thing I should warn people about is that you can still die by close quarter combat kills and if that happens you'll have to exit the game and restart as pressing restart doesn't do anything. The AI just keeps playing but won't go to a checkpoint.
Also, I dunno if this is just a glitch in the game or something the trainer causes but the sound goes out alot. It happens at least 2 or 3 times per level.
@RedPanzer, you may just have to wait till Steam rolls out the update to your country. I know super geek has the retail version and he was playing online with version 1.0 all day till a few hours ago and he told me it kicked him off for the update.
I dunno why the updated trainer isn't working for him now though.