I dont read cd reviews. Ive seen Saul Williams and NIN live and if they were to try collaborating in that respect it probably would be a trainwreck...but, again, the beats that Saul raps over on his own records are usually kinda grungy/rock-y and similar to the music that Reznor can write well. As far as I know that is all that record will be...Reznor making music and producing with Saul Williams rapping as per usual. It seems like a very fitting combination. I also think Saul might work well with RZA.
If Reznor does intend to put himself at the forefront of the Saul cd by doing some vocals or making overbearing music...then, yes, it will obviously work out poorly. But if it remains as reported and Reznor just keeps to the background then I see no problem with it.