Let me put it this way. Despite what any of us may think, it will be upheld as law, and be in full force and effect, if the Supreme Court thinks it's Constitutional.
"For simplicity's sake, I'll just say that I feel that Fast Track sidesteps the Constitutional requirement of advise and consent by disallowing the submission of ammendments."
That's my main problem with it. The original plan was great: the President negotiates, but keeps the Senate informed all the way and listens to their concerns, so that when the time came for ratification, it was a shoo-in. Either that way or Fast Track will give the one advantage I think Fast Track has, which is that when we negotiate a treaty with a country, that country will know at least that it's either a done deal or not -- not if it'll get torn to hell by a bunch of special interests.
That said, I think we've established that Fast Track will not necessarily deal with treaties, but simply trade agreements, making any of our above concerns moot as far as Constitutionality goes. Instead it will deal with a quite Constitutional (according to precedent and Constitution) ability of the Congress to delegate its authority to the Executive Branch. In this case, it's the authority to control commerce with other nations that it's delegating (well, not completely since they can vote it down).
More reason why there needs to be a Constitutional amendment mandating periodic Senate reconfirmation for the Justices every 8 years, and Congress needs to restrict the Court's appellate jurisdiction in areas it has been known to legislate from the bench in.