The reviews already posted here are largely accurate -- the translations supplied by this dictionary are short and not as complete as you'd expect from a real French-English dictionary. However larger dictionaries typically cost more than this anyway, so I don't think this should surprise anyone. Also, many forms of words that are in the dictionary will not be recognized by the dictionary in the auto-lookup default dictionary mode (including irregular conjugations and words preceded by l' (eg. l'ombre, as one reviewer pointed out).
There's a lot of room for improvement, and as soon as another French-English dictionary is available that works as a default dictionary, I'd recommend switching, unless the index for this dictionary is improved to address these problems.
However, I'd like to clarify a few things --
First, stemming does work in most cases, and you can look up conjugated forms of most verbs without trouble. I was afraid before I purchased it that I'd only be able to look up singular nouns, infinitive verbs, and masculine adjectives, which isn't the case. This means that most of the time, you'll be able to look up words instantly. This is a *huge* improvement from looking things up in a separate dictionary, which is great for the price. It is of course annoying when a word won't auto lookup, but then you could look it up manually in the dictionary.
My recommendation is this, at least until a better french-english default dictionary is available:
Buy this dictionary. It will *dramatically* improve your french reading experience on the Kindle. It has flaws, but they're not worth worrying about at the price it's at.
If you're an academic or an expert, you'll probably find it annoying sometimes, but still helpful. Buy a better dictionary. But I'd probably also recommend buying this one, if only for the convenience of the auto-lookup feature.