I bought the film specifically for the Gene and Jerry "Worry Song" dance. At the time of its release, Walt Disney said the blend of animation and live action was more advanced than anything being done at the Disney studio. The only complaint I've heard regarding the sequence was that Jerry was too large, closer to the size of a Teddy Bear than a mouse. Sure, he's a little gargantuan for a mouse, but I can suspend disbelief for the sake of a brilliant musical number (besides, I fully understand why the animators made him the size he was).
This scene, along with the lively "I Begged Her" Kelly/Sinatra dance duet, are the best sequences in an otherwise ho-hum MGM musical.