Works great on my Android Tablet. I don't know about Apple. Book selection is also very good at reasonable prices, with several classic titles available. If you already own a tablet, just by installing the app you can see what is available. If you don't already own a tablet, you can use their website to see what is available (on Android).
Experiences with Gambit "Chess studio" app?
Also, rereading your question, you do not seem to realize that there are two other free apps, sponsored by Forward Chess and by Everyman, that also work great with Android. The 3 apps combined cover a lot of books, both recent and historical.
I use 2 chess ebook readers - Gambit Chess Studio (GCS) and Forward Chess (FC) - for almost 2 years and practically stopped using paper books. GCS is far behind FC in almost every aspect (but I am a big fan of Gambit books)
Here are some points of comparison:
- GCS can be used for Gambit books only, FC collaborates with many publishers - Quality Chess, New in Chess, Russell etc.
- GCS has beautifully designed text part, but board design is rather poor- only one size (too small), one color, one piece set. FC has 3 board sizes, 4 colors, 2 piece sets.
- FC can work in both landscape and portrait mode, GCS - only landscape.
- FC make few releases per year, it takes GCS 1-2 years to release a new version.
- GCS has some useful buttons (missing on FC) - forward/backward, next/previous game
- FC has bookmarks, GCS doesn't
- In FC you can organize your books in folders, not in GCS
- FC allows you to make moves on board, GCS doesn't
- FC has build in Stockfish engine for analysis, GCS doesn't
- Both apps allow you to Copy a position as FEN (available on Android, not sure about iOS). This is a fantastic feature allowing you to Paste it into other chess app and play against computer or analyze. However GCS has an annoying bug when casling flags in FEN are often wrong, I reported it more than 1 year ago but it was not fixed. With FC bug reports and feature requests were replied within days and in general comuunicating with them was much nicer experience
- via web shop FC allows you to share books between Android and iOS. For examples I've purchased all my books for Android tablet, but when web shop appeared I could freely download same titles on my iPhone. Very nice. I doubt GCS has something similar.
- GCS books seem to be cheaper on average

Has someone used that app? Is it good?
It's basically a mix of an ebook reader and a board, showing the moves from the book, so it might be a nice addition when travelling (train or plane for example). It's limited to their selection of books, but that would be fine, as they have some good reads.