I think some of the negative reviews here are from people who are trying to use this dictionary in ways for which it wasn't designed: so it's not a French phrase book, it doesn't have extensive 'tourist' vocabulary (e.g. for reading restaurant and café menus), and it's not ideal, though perfectly possible, to use it as a standard dictionary where you input a word in French and the English translation pops up. Where it comes into its own is if you are reading French texts on your Kindle and need an instant translation of a specific word - a sort of quick and dirty translation/reminder.
As one reviewer here notes, it won't translate the odd French word in an English text: Kindle recognises that the majority language is English and so automatically looks up the French word in your English dictionary. As long as the language of your text is French, however, then this works fine. I read quite a lot of French novels and there are always words that I've forgotten or don't know - and this is perfect as an instant look-up tool. It works the same way as the Kindle English dictionary does: just touch the word (or move the cursor to it on the Kindle Keyboard) and the translation pops up.
This recognises some inflections, though not all, and the translation/definition is brief and basic: perfect for reading novels. I also have the Kindle standard French-French dictionary in use and so if a word is not recognised in this French-English dictionary, Kindle automatically defaults to French-French which usually sorts out my problem.
So this is a good tool if you already have relatively good French and read a lot of French Kindle texts - it won't replace a proper dictionary but it's an excellent way of looking up words when reading on the tube without breaking the flow of the text.