In case it hasn't been mentioned, in my AT&T 16GB GS4:
9.72GB is remaining for user use.
I have about 199 apps installed and now I have 6.93GB free. According to the phone, 2.04GB is used for apps so the rest is used for miscellaneous stuff.
I am not a big gamer and really at most would have a few games so this would work for me. For people who install a bunch of 1-2GB games, though, I can see this being an issue.
Pretty happy so far and I do love the screen (my primary reason to upgrade from GS3)... I wish my case would come so I'm not as terrified when holding onto it bare.