"The Mammoth Book of Monsters" is a satisfying collection of monsterrific tales of various types. Not every one is a hit, but if you buy this book you will undoubtedly find a few that tickle your fancy. I may as well tell you what my favourites - and least favourites - were.
I particularly enjoyed three stories, which also happened to be some of the longer ones in the collection. The first is THE HILL by Tanith Lee. The language this story is written in is bizarrely old-fashioned given it isn't THAT old, but it is genuinely disquieting and original. The second story I liked was THE FLABBY MEN by Basil Copper. This is a slightly sci-fi story about parasitic blob monsters on a government research island. And the third story, which was probably the best in the collection, was Clive Barker's RAWHEAD REX. Rawhead is an original monster, and this story devotes a good deal of time to his point of view, which was interesting, and it reads quite like a mini-novel, so complete is the story Barker concocts. This story is what this collection is all about.
Other stories which intrigued and entertained me, but not so much as those listed above, were DOWN THERE by Ramsey Campbell (takes a while to get going, but suitably horrific by the end), THE HORROR FROM THE MOUND by Robert E. Howard (a vampire tale from the 1930s which reads surprisingly modern today), THE THIN PEOPLE by Brian Lumley (a weird, largely goreless and non-violent tale, but fun nevertheless), OUR LADY OF THE SAUROPODS by Robert Silverberg (bio-engineered dinosaurs...inspiration for "Jurassic Park", perhaps?)and SOMEONE ELSE'S PROBLEM by Michael Marshall Smith (in which a man sees - or thinks he sees - monkey-like monsters on a train; has a bit of the feel of the classic Twilight Zone episode starring William Shatner on the plane to it). All of these contain fun takes on monsters, although only the first two could be truly said to be horror stories.
As for ones to avoid: don't bother with VISITATION by David J. Schow. It has a reasonable premise - that of a certain hotel which acts as an inter-dimensional gateway for various nasty creatures - but it overwhelms the plot with tons of spiritualist mumbo-jumbo that turned me right off. Also steer clear of Scott Edelman's THE MAN HE HAD BEEN BEFORE; it markets itself as an apocalyptic zombie story, but is really just a story about a kid with a mean daddy. I'm not averse to having character drama and human monsters in stories, but in this case the zombies might as well not have been there, rendering it largely pointless in this collection.
Any other stories that I have not mentioned here (there are eleven others) are ones that simply made no impression on me at all. You may enjoy them; you may not, but I think my above lists of the ones that I enjoyed are sufficient to allow you to decide whether this collection is your cup of tea or not.