Yes, it is worth watching.

Thief set the groundwork for future Mann projects where he had greater success, after watching you will see where Miami Vice (TV show), Heat, Collateral, Manhunter and Crime Story derived their roots in story and format. Producer Jerry Bruckheimer was also involved setting the stage for his partnership with Don Simpson and their meteoric-rise in the 80's with slick made movies with distinct lighting, notable background music and catchy soundtracks.

This movie was also the launchpad for actors James Belushi, Dennis Farina, William L. Petersen and notable character actor Robert Prosky

Great flick. I watched it yesterday on youtube, full movie available for free with commercials.

Also came across a documentary type thing on the making of Heat which I thought was really good.