I saw this exhibition when it was on at the Clore Gallery a few years ago and only had access to a copy in the University library when I was at art college so wanted one ever since.
It's a comprehensive record of the exhibition, good quality pictures and lots of them. The text accompanying the images is useful, explanatory and not too intrusive.
The down side is that some (though not many)of the illustrations are black and white which is annoying as Turner's work is a lot to do with colour. That's only a small moan though.
Overall, if you are a Turner fan or just like landscape or watercolour paintings, buy this book.