I went out and bought it at once (for some reason it was on sale from day 1), but I only just started reading it a few days ago.
It is alright so far, but it is not like there is anything exeptionally new about it. . . what it does is that it makes the story for Turin easier and depending on your mood more enjoyable to read.
Now you don't have to read the UT and the Sil at the same time