A competant enough translation, not the best or the worst that is out there. I can't say yea or nay about how Lady Charlotte Guest did with the source materials of the book, but I have to say that it's pretty entertaining.
It chronicles various tales from Wales, going back to ancient legends about heroes and gods, and then forward in time to recognizable elements such as King Arthur. Since this is a Dover thrift book (one of the pricier ones, since it's longer) the quality is so-so; the cover artwork is better than most of the thrift books, and the paper is only a little better than newpaper-quality.
In terms of readability, however, Guest's Mabinogion does not win any prizes. The translated sentences really could use a little tweaking, and often dialogue between several people is crammed together into one long paragraph. This is not only distracting, but hard to read.
Overall, it is fairly nice but needs an editor to tweak it in places. Fans of fantasy as well as mythology may want to check this out, especially if they are fans of the Prydain or Lost Years of Merlin series. For more readable Celtic myths, try Ella Young's "Celtic Wonder Tales."