Rashi (according to the Medrash he brings on posuk hey (Gen. 40:5) says "tov" here means "correctly". This is because each one dreamt his own dream (without its interpretation) and the correct interpretation of his friend's dream as well.
The Ramban says both pshatim. He brings Targum Unkelos who translates "tov" as "nicely". That means to say that the baker would never have told Yosef his own dream unless he witnessed how Yosef knew his stuff well. :) OR it can mean he interpreted it favorably (tov).
The Seforno says it means that the baker saw Yosef give a favorable interpretation and thought he would get one too so he also told him his dream.
The Rashbam holds it means the baker saw the words were true. (Tov means he interpreted correctly.)
I believe the Ohr HaChaim says like Rashi about the Medrash (he saw it was correct because he had knowledge of the correct interpretation), and also brings the pshat like the second answer in the Ramban and the Seforno (tov=good and not bad.)
Apparently it lends itself to both.