I was skeptical at first, I've seen a lot of attempts over the years, but holy shit, it works (and on Linux I might add).

Links to download are in the article. It's not seemless, but the world is "alive". Drove around, got a 5 star wanted level, died and respawned, people out walking around, rode the subway, it was great.

Things noticably not there - spawn locations of your favorite cars/helicopters, and of course no story stuff - but I spent a few hours having a blast.