I never had a need to use a phrase book until I made my first trip overseas. Mind you, by first trip was to Hong Kong, which meant that I went out and purchased a Cantose phrasebook. Well, it turned out that I didn't actually need it, namely because a friend of mine met me at the airport (quite by surprise mind you, since I didn't expect him to make an appearance), and Cantonese happened to be his native tongue, which meant that he acted as a translator for the entire trip. However, when I returned to Hong Kong a couple of years later, I took that phrasebook with me, and it turned out to be completely useless – I didn't know a word of Cantonese, and as it turned out, I couldn't even pronounce anything without mucking it up. Mind you, with the exception of cab drivers, I managed to survive for the week simply by pointing at stuff (though when it comes to money, they all know what I'm saying).
So, I have come to the conclusion that phrasebooks can be very useful – if you already have a basic understanding of the language. I discovered this on my last trip to Germany because since I know how to construct a basic sentence (which generally involves ordering a beer, or telling somebody that my German is pretty shocking), I have an understanding of the phrase that the book recommended. Since I am now on my way back to Germany (and have been studying it on an off for the last three years, with the help of Duolingo over the last six months) I'm going to see how much better I am with it now (though I still have my phrasebook).
As for French, as a language it actually isn't all that bad. Ignoring the fact that adjectives, demonstratives, and articles agree with number and gender (though nowhere near as bad as German), the grammar is actually pretty similar to English (with the exception that a noun can't exist without an article – for instance you can't say I eat eggs, you have to say je mange des oeuls (or I eat some eggs) – there has to be an article in front of the noun, otherwise it is bad grammar. Oh, and don't say je suis chaud because, like German, it doesn't mean I am hot it actually means I am horney. So, ignoring vocab (which is the hardest part of any language), grammar in French isn't all that bad.
However, there is a catch – as a written language it is absolutely shocking. You probably know that there are a number of English words where you have a bunch of letters (such as cough) that aren't actually pronounced? Well, in French that is basically every single word. For instance you never pronounce the last letter in the word, unless of course you have to (for instance, if the next word begins with a vowel or an 'h', then you have to pronounce the letter). This means that plurals become a right pain in the neck since to turn a noun from a singular to a plural by adding an 's', but you never, ever, pronounce the 's' (unless of course you do). So, you have to get around that by using other words (such as beaucoup - a lot; or quelques - a few).
As for this book, it pretty much covers everything, right up to chatting up somebody at a nightclub. Mind you, if you have to resort to a phrasebook to chat somebody up then you are probably not going to get anywhere anyway, and I'd probably recommend sticking to your native language (unless you have an eidetic memory, but then again if you do, you probably don't need a phrasebook). In fact I'm still trying to work out if parts of the book are actually being a little tongue-in-cheek, or if the author is deadly serious. Personally, I have never viewed the Lonely Planet brand as having an element of tongue-in-cheek, but I'll just assume that the chapter (or section) on sex is just that.