Mos Def is pretty solid, he's a schooled actor after all JOURNEY TO THE END OF THE NIGHT with him and Brendan Fraser
http://www.imdb.com/title/tt0454879/ is a movie I highly recommend, it's a gritty street-thriller where Brendan Fraser does a very different role he plays a pimp who's looking for a way out, Mos Def is a guy who's involved in the plan for him to get out... It's reminiscent stylewise of COLLATERAL and CITY OF GOD...
Seen him also in BE KIND REWIND, ITALIAN JOB and 16 BLOCKS he brings alot of warmth to his characters, although I do believe that he has more potential...