Learning from a phrasebook has always been a waste of time for me.
A bit/quite a lot of the language learnt before the trip is my way. The phrasebook is the secondary aspect - taken along on the trip - there for those situations you haven't mastered the vocabulary for/the stressful moments when your mind goes totally blank.
I find most of the translations in phrase books to be unnecessarily complex for someone without knowledge of the language. It's more useful just to have simple templates and essential practical vocabulary. When speaking a language I don't know, I deliberately keep my questions very simple. With luck this will encourage people to keep their answers simple, so I can understand them. To ask for something in a shop you only need to know the word for the thing you want, for example. Another thing is that they are often full of errors, as if the authors' only aim was to produce a book's worth of phrases as quickly as possible.
To ask for something in a shop you only need to know the word for the thing you want, for example
If you can see what you want, you only need to know the words for hello, goodbye, please and thank you
The Dorling Kindersley visual phrasebooks are great, by the way. Our Italian agent was so taken with the one I bought in Ravenna that he kept it to hand out to those Anglophones he deals with who normally don't make an effort, so he doesn't have to translate for them in restaurants all the time. Note the "You may hear .." section.
I've also found phrasebooks to be good ice-breakers. I've sat alone at bars in Italy, Kazakhstan and Russia and had staff and other customers, locals, ask to look at my phrasebook and amuse themselves over the translations
I use phrasebooks a lot when I'm travelling, as an aid to using and yes, learning, the language. However, I always start by trying to get a grasp of the basic grammar of the language from other books before I go ( although the best phrasebooks do have a grammar section - shame Berlitz dummed down theirs).
This basic grasp means that I can then use the phrasebook as a huge body of examples of the language in use, and I hope to pick up a set of model sentences that can be used in different situations and with different vocabulary - which is where the vocabulary part of the phrasebook is essential.
A good phrasebook will often suggest more natural ways of saying things, which may not be obvious from a study of grammar only, eg. if I want to say ' is there an XYZ round here' it is better to have a dozen examples in the phrasebook than work out for yourself how you will translate 'round here'.
As for understanding replies, a phrasebook is little better than any other method. You're a learner, it'll be difficult at first, and the more you've studied in advance by whatever method, the better equipped you'll be. I would never hand over a phrasebook and expect the native speaker to point out a reply, but that's just me.
So I suppose to summarise, a phrasebook will rarely be a starting point to learning a language, but it can be a very valuable aid in the learning process. And great fun.