I would say that it's value is symbolic. What it now represents as part of our American history is important, but as others have said, constitutional ammendments have superceded this document. It simply serves as a symbolic reminder.
Important in and of itself, not really, since at the time, it didn't even free any slaves. Historically, however, it is very important, in that it marked a turning point after which slavery was doomed, and in the short term. Government, for the first time in its history, had taken an official position against slavery, one which was irreversible.
It's also important when we study Lincoln, because both the political genius of the document, its timing, and the way in which it is worded tells us a lot about Lincoln as a President and a person.
This is somewhat like the Gettysburg Address in that it's not really important in any real terms. There are no areas in rebellion against the federal government and there are no slaves. So the Proclamation has no real impact today. Once the 13th Amendment was passed, the Proclamation had no further legal importance.
Symbolically, it is somewhat important, though I would say that it is not as important as the 13th Amendment itself. The Proclamation was so much weaker than the 13th Amendment because it only freed slaves in areas that were rebelling. So I would say that this is not really that important today except as a historical document that tells us about Lincoln's thinking.