NOT A TEACHER
(1) I think that you can exclaim:
What a good/great song "Until You" is!
(2) I think that there is a difference between "What a good song [it is]!"
and "This song is so good."
(a) I believe that when you use "so," it is usually necessary to complete it
with a that- clause:
This song is so good that I want to listen to it 24 hours a day.
This song is so fantastic that I forget everything else.
This song is so awesome that I hum it everywhere I go.
Student or Learner