This phrase is typically used to introduce a fact or piece of information that the reader should take note of. For example: "Important to note is that the deadline for submission is fast approaching."

"of important note" is a correct and usable phrase in written English. You can use it when you want to emphasize something as being especially remarkable or noteworthy.

"It is important to note" is correct and usable in written English. You can use this phrase when you want to emphasize the importance of something in writing. For example, "It is important to note that the deadline for submitting the project is tomorrow.".