The new "Neo-Humanist Statement" on Paul Kurtz's website is in many ways a good attempt at a synthesis of what I and most of my friends believe and would want to support. Importantly, it contains a very useful let-out clause that I recommend to people who are drafting manifestos: "Those who endorse this Statement accept its main principles and values, but may not agree with all of its provisions."
With that in mind, you may want to sign it. However, you might also want to consider the counter-argument over here, from Ron Lindsay - and the comments following. In the end, I'm with Lindsay. I could just about agree with the statement's "main principles and values", but there is just too much detail that might come back to haunt me. In particular, it is too specific in its plans for international regulation and is too negative about the current "New Atheism". Given the let-out clause, I might have been prepared to sign something shorter, less specific, and less carping about the efforts of allies.
However, my readers will want to be aware of such developments. By all means sign the statement if you think you can do so without too many reservations (and bearing in mind the let-out clause).