Just watched for the first time
I'm a huge fan of Michael Mann's work. I count Heat as among my top three films of all time. Somehow, up until now, I'd not seen Thief. I picked up the Criterion bluray a few weeks ago, and let me just say, people aren't exaggerating when they call this film a masterpiece.
Thief has everything that made Heat such a great film, i.e. strong character development, action, realism, atmosphere, and plot. Thief shares a lot in common with the look and feel of the Miami Vice tv show, which Mann would go on to make a few years later.
Highly recommended watch.