If you mean predictions as to "what will actually happen", this depends heavily on the outcome of the current negotiations, and I don't think anybody has a reliable idea at this point.

If you mean predictions as to "what would happen if absolutely no change to current law was made", then I think the standard prediction is that there would be a negative hit to the US economy from the "austerity", pushing it back into recession. But this would happen over a period of several months, not right away on January 1st, leaving quite a lot of time for a compromise changing the laws to be negotiated. So there is no immediate reason for alarmism. In addition, this prediction assumes no extra non-legislative policy measures, such as new monetary stimulus by the Fed, that could potentially soften the impact of the cliff.

I don't think there are any predictions that can be identified as very reliable a priori, to be honest; economics is hard, and economics combined with politics is super hard.