Oxford beginner's Japanese dictionary is pretty good on the whole for the aspiring student of Japanese, but is by no means perfect.
On the plus side, all of the Japanese is in kana (you *need* to know kana if you ever want to learn the language properly), and the most useful/common kanji are given next to each translation. The selection of words matches the needs of a beginner well, meaning that you don't have to trawl through endless pages of useless vocabulary to find what you want. There are also grammatical examples to accompany some of the translations; however, some of these are quite advanced, and since a literal translation is not provided they aren't quite as useful as they could be.
There is a useful grammar section in the middle, which contains tables of verb and adjective conjugations, as well as pages of references for family names, dates etc. However, no explanation is given to what the different verb forms actually mean/do - what exactly does causative passive mean, and how is it used in Japanese? The reference tables for verbs that I'd covered already in my studies were useful however, but clearly the grammar section is intended as a supplement rather than a tutor in it's own right (for wonderful, clear explanations of all Japanese grammar see 'Japanese the Manga Way' by Wayne Lammers). Furthermore, at the back there's an extensive phrasefinder. If you're just want to use it as a phrasebook for (say) a holiday to Japan, I suggest you look elsewhere - as with the rest of the book, no romaji are on offer. If you're actually learning the language though, they serve as a useful reference point.
Some of the translations provided are quite ambiguous - for example, 'ginkou' is listed simply as 'bank (n)'. Were it a word you'd never come across before (and there are other examples), you might be left wondering if it was a money-bank or a river-bank. Additionally, after fairly extensive usage, I've come across quite a few mistakes/typos. Obviously it's never going to be perfect, but it doesn't exactly inspire confidence.
The book is attractively presented, and is beautifully free of off-putting, detailed linguistic analyses. Entries are written in a bold blue font, whilst translations are in black, so it's easy to find whatever you're looking for.
So why four stars? It's partly because I have yet to find anything better (or at least, anything that doesn't have romaji all over the show), and also because I'm really nitpicking. For most purposes it's a useful reference, but you really need a teacher to clarify the various ambiguities within to make the most out of it.