Katherine Hepburn would seem ideally cast to play a wildly talented woman (Clara Wieck was one of the great 19th-century pianists, an amazing feat in an era when women were not supposed to have "careers") who has to break away from her father to lead her own life.
In some ways, Katherine Hepburn's performance as Clara Wieck is one of her best, simply because she has relatively weak material to work with, and her ability to give it life becomes apparent -- she brings real passion to what, from another performer's mouth, would sound silly. Ditto for Robert Walker, whose Brahms is self-assured and even a bit wise-ass at times, not far-removed from the real Brahms. Both take trite and thrice-heard dialog and give it imaginative treatment.
"Song of Love" makes lavish use of Schumann's music (mostly piano -- his orchestral works aren't even acknowledged), with outstanding performances by an uncredited pianist, and the MGM house orchestra conducted by William Steinberg, who went on to conduct the Pittsburgh Symphony. The performances are _so_ good, I'd like to hear the complete versions (if there were any) apart from the film.
Amazingly, Hepburn, Henreid, Walker, and Daniell all had some degree of piano-playing skill, and we see them actually tickling the ivories in a convincing fashion. (Some of the playing appears to be undercranked, so it looks as if Clara's or Franz's fingers are flying across the keys. The film implies that Clara was nearly as good a pianist as Liszt.)
Dramatically, the film is all over the place, with good scenes (Wieck pere telling Bob why he shouldn't marry Clara, Clara mouthing off to Liszt about his interference) followed by cutesy Hollywood creations (Clara encouraging Bob & John to kill a chicken for New Year's Eve dinner). The movie's principal failing is its shortness -- we never see Bob & Clara actually falling in love -- and the inability to move Bob past the point of The Tortured Composer Without Recognition Suffering From Mental Illness Of An Unidentified Sort. Bob & Clara had an intense sex life (Clara marked her journal to indicate when they had sex -- there are a _lot_ of marks), and the film suffers (as, oddly, "Brokeback Mountain" does) from the lack of an intense and passionate scene of love-making. There are times when sexual explicitness /is/ appropriate.
As a classical-music lover, I bring an interest and prejudice to this film the average viewer lacks. (I cried at a few spots, mostly because of my fondness for the Schumann-Ruckert "Widmung", which gets heavy use.) How they will react to this film, I don't know. But it's worth seeing to hear the excellent music and to see how fine actors handle less-than-great material.
The "triangle" among Bob & Clara & John is a fascinating subject and perhaps an adventurous filmmaker will someday create an "Amadeus"-like film about it. In the meantime, you can enjoy Jan Swafford's excellent biography of Brahms.
