I've just started to learn Welsh and am finding this dictionary (ISBN 0-19-864531-7) a huge help.
Contents are clearly written and explained, and example sentences or phrases of the word in context are given.
For nouns, the plurals and gender are specified, and all words affected by, or causing, mutations are identified too.
The dictionary is very clear and helpful in the way it is written and presented, and is printed in a clear font on good quality white paper so it is easy to find and read definitions. The mutation and grammar guide are also useful, and there are also lists of countries and towns/cities in both languages.
What lost the dictionary the other 2 stars for me is the way the pages are fastened to the cover, i.e. with only the briefest touch of glue. Within 2 hours of using the dictionary (and it wasn't open flat to strain the spine as I'm very careful with books) the pages started to come out. I already have a pile of loose pages and these will get creased and tatty even if the rest of the pages stay stuck to the cover - and I have no doubt that that won't be for long. It is the most badly constructed book I have ever owned.
I've supplemented this with Collins Spurrell Welsh Dictionary (ISBN 0-00-719658-X) which has over twice as many entries, but only a straightforward translation with no context (e.g "bow" is bwa or dolen, but is either of these a tied ribbon or the front of a boat?) and the two are working well together for me. (Incidentally, this book has a waterproof cover, is smaller, thinner and of way better construction despite costing only £2.49 including P&P)
In summary, I'd highly recommend the content of this dictionary for Welsh learners, just not the very disappointing build quality.
I've now received a replacement for the book - the later edition (ISBN 978-0-19-922874-4) which uses slightly thinner, lower-quality (less white) paper, but seems to be constructed much better, so I'm confident this won't fall apart. The content appears to be identical to the previous edition, but as a result of the paper quality change, the book is slightly thinner and lighter making it a little more "pocket" sized, even though that part of the title has been dropped. The addition of a shaded letter of the alphabet being printed on the edge of each page makes searching for a word quicker. I'd give the later edition the full five stars!