There is 100% chance of cliff being avoided IMO. However, both sides have to look like they went to the mat for their constituencies (just like we saw in Greece and Spain), so it will be in the final hour, or shortly thereafter. Then, each will claim they only compromised in the best interest of the American people because the other side would have taken them over the cliff had they not.
Best case, I think we go into that weekend without a deal and may get one Sunday night of the 30th before the markets open on the 31st (if they are open that day). Otherwise, I wouldn't be surprised if they wait until the night of January 1st, since the markets will be closed anyway on the 1st I assume and it really makes it look like the negotiated as hard a possible. Of course, I hope I am wrong. I hope we get a good deal soon so my longs will go up, but I doubt it.