I have not seen The Soloist yet, but based on an article by Tara Dooley of the Houston Chronicle Jamie Foxx does a believable job protraying a Juilliard trained cellist in this new film.
“He was very convincing,” said Norman Fischer, cello professor at Rice University’s Shepherd School of Music, a premier Texas music program. “To do what he was doing was very impressive.”
There are some moments in the film where a professional might recognize the sound we're hearing isn't actually coming from Jamie's playing, but it's done well enough to be a creditable performance.
Of the 43 news articles posted about the film, most were only luke warm about the overall film. Some suggest it is "cheesy and overdone", other's think it was trying to be too much for one film. While the music is generally thought of as wonderful, the general attitude is The Soloist missed the mark.