I'd suggest that this is the best annual anthology of science fiction - there are other worthy annuals, notably Gardner Dozois also edits "The Year's Best Science Fiction" and David G. Hartwell & Kathryn Cramer offer "The Year's Best SF". If you're keen on science fiction and the short story mode, then all of these will provide a range of science fiction showcases the work of the best contemporary writers. As is inevitable in any such anthology, there will be some stories you enjoy more than others - it can be a curate's egg of an adventure.
So what makes the "Mammoth" book stand out? Well, it contains a 50-60 page introduction which reviews the last year in science fiction - novels, magazines, television, cinema. It's a fascinating read for the science fiction, and for the would be writer, an essential one. As a professional writer myself (though not of science fiction), I would regard this is an ideal package for anyone wanting to write in the genre - it contains a wealth of practical information, plus evidence of the standards you need to achieve to become a successful, published writer.
The "Mammoth" offers excellent value, and that has remained my consistent opinion for years. What more can I say?