Let me start by saying I'm reviewing THIS EDITION not the story. I'm assuming that you have read at least one book by DWJ previously, and already know she was a very good author.
It's quite good having the two books combined into one volume if, like me, you're a voracious reader and will finish one of these in under a day. It also means it takes up less space on your bookshelf (only marginally, but if you buy the entire Chrestomanci series in double-books then you're looking at saving the equivalent to one additional (small) book's worth of space on your bookcase, because there's less covers and less of those bits of empty paper/copyright pages etc etc at the front and back of the books). I would say this is definitely worth the money and the art is better than the last edition (which looked like it was aimed at 6 year olds) so I don't look silly pulling it out to read on the train.
As for the actual story - I really don't hold with posting spoilers on reviews, but if you like that sort of thing I'm sure someone has summarized the entire plot and all its minutiae. Someone always does.