An awkward one to review in some ways. What am I reviewing? Armitage's editing and rendering of the text into modern English? The balance of his poetry as he does so? The original story?
Possibly all of the above. The basic story is very simple. A big green bloke wanders into Arthur's court, issues a challenge that he'll let someone take a swing at him with a sword if he then gets one in return, and goads Gawain into accepting. By fourteenth century standards it's actually quite good. Certainly better than the anemic tripe that is Merlin, the BBC's take on the Arthurian story.
Poetically, the heavy alliteration can be annoying at first, but that's more medieval poetry's fault than Armitage's. There's a reason Shakespeare made such fun of that sort of thing in A Midsummer Night's Dream. If anything, this slight irritation shows just how well Armitage has captured the spirit of the stuff.
I'm probably being a fraction harsh anyway, since mostly the piece is wonderfully unobtrusive. The trick seems to be not to read it slowly, as with poetry, but at a normal sort of speed, letting the poetic devices and the rhythms of the language do their work unseen. Read like that, this is really well worth reading.