Well, I wouldn't want to pay £2.99 for it - but then I hate paying for Apps (or e-Books) but, I guess, it is worth having for free on one's iPod and iPad for immediate reference without having to connect via 3G or Wifi. But, beyond that, there isn't really much "added value"
A couple of immediate points
a. It is only up-to date as far as 2010 inscriptions. Whether they intend keeping up to date I don't know - I think the Collins book has only been updated once since publication and this probably has to wait for any further book update.
b. A surprisingly (and disappointingly!!) large number of the sites have no photos on the App - just a map of their location, together with the instruction "View available images on the UNESCO website" and an arrow linking directly to the Web page - IF of course you are connected to the Web!!!! This seems somewhat to defeat the object of the App (immediate off line access to the information)! As far as I can see, on every one of these sites without a photo on the App, the photo on the UNESCO Web site is has a limited copyright (including in some cases with UNESCO itself and sometimes jointly with a UNESCO employee). It would seem that those "rights" do not extend to including the photo(s) on a downloadable App. I find it amazing that UNESCO doesn't have a FULL set of photos of every WHS for free use - you would have thought that it would have insisted that every States Party provides such a set of photos. That also explains why Collins had to search around the web sites of stock photos (with mixed success) to find photos for its book. Given that both the book and the App have been produced with UNESCO support (and, presumably, some financial return??) you would have thought UNESCO would have provided the photos!!