Tough construction: Are these sentences all correct?

[October wind]

Are they all correct?

A) The tool is not only easy to use but also nice to look at.
B) The tool is not only easy to use but is also nice to look at.
C) The tool is not only easy to use, but is also nice to look at. (With a comma)

 

[emsr2d2]

C doesn't work because the "not only but also" phrase is a set phrase and it doesn't involve a comma between the two halves.
A and B are grammatically correct. The second "is" in B is optional. It's unnecessary but it doesn't make the sentence incorrect.