Rambling nonsense when he steps beyond what he knows. 
 Most of his ideas on the "tree alphabet" are his own and sourceless. Unfortunately a lot of the celtic magic industry owes too much to this as a gospel of sorts. Better and more scholarly book are out there if you can be bothered looking. But they are without the glamour of Graves which I suppose is part of the attraction to the sidhe huggers. 
 Edit: 
 This is a dreadful book...yet I've read it twice, the 1st time in the early '80s and again in '13. It hasn't gotten any better. On the re-read I read more critically and cross ref'd him. After the 1st read I had a bee in my bunnet about his made up tree alphabet nonsense (see above review), on the 2nd read I now realise he just made up pretty much everything else too. 
 Graves (and I am a fanboy for most of his other work) comes across as an arrogant arse. 
 He seems to use the No True Scotsman attitude when discussing poetry (http://en.wikipedia.org/wiki/No_true_...). He discounts any poetry as not being real or true poetry if it doesn't conform to his standards. He seems to be saying you have to be of celtic stock (whatever that means) and a heterosexual male to be a poet...and only if you write on certain themes and have muse like inspiration at that...'cause that's all women are good for...well that and the orgies and temple prostitution obviously. 
 And he makes no mention of Burns or Yeats who pretty much fit his definition (sour grapes on his part?). 
 I like his fiction but I have the feeling he was a bit of a dick. 
 But this book isn't really about poetry or history....yet it claims to be. The historical evidence is hammered and moulded to fit his hypothesis with contradictory ideas ignored or glossed over. Graves bases a chunk of his arguments on a re-ordering of the old Welsh poem Cad Goddeu, yet he has no Welsh and uses, in his own words, "D.W. Nash's mid-Victorian translation, said to be unreliable but the best at present available." 
 Then he goes ahead and juggles the order. 
 See what's going on here? 
 He's making it up based on a poor translation, hammering the facts to fit his hypothesis. 
 If someone had no understanding of any other text's original language and then used a poor translation prior to re-ordering the entire text to fit an idea would we be as tolerant? 
 I hope not. 
 And this technique isn't limited to this one poem, it is his default method. 
 He says, amongst other things, "this must be a mistake" and "a stanza has been suppressed" when he isn't getting the confirmation of his hypothesis he wants. He has re-arranged entire poems. 
 There's a lot of "perhaps", "likely", "seemed", "suggests", "obviously", "evidently", in this book...they miraculously transform into what Graves sees as solid fact, at the end of ch.7 "conclusive proof". 
 Yet he has claimed "fact is not truth, but a poet who willfully defies fact cannot achieve truth." 
 Sniff, sniff...what's that smell? 
 Also, he seems to show little demarcation between deities (x appears to be y, who is actually z, but on closer inspection is really the same as a, who was worshipped as b etc etc etc). This syncretism is all well and good, but when the Venn diagram of deities ignores everything but the bits that fit his hypothesis and the focus is purely on the overlap I start to smell shite in the argument. 
 I'm with Francis Bacon when he says: 
 "The root of all superstition is that men observe when a thing hits, but not when it misses." 
 Rambling, havering keech...data ignored or manipulated, poems re-ordered, poets sneered at, women paradoxically praised and dismissed in equal measure...had he just been chucked by his wench/bird/muse prior to this or something? 
 I got the impression Graves was a bit xenophobic, homophobic and sexist (OK, I don't know if he actually was homophobic etc...but he occasionally makes comments that are a bit off). Now there may be a part of this due to him being a man of his time...but considering the hypothesis in TWG it jars a bit. But there's an irony here though isn't there? That a foundation text for modern wicca and neo-paganism, both fairly female friendly, is this way. How do folk square it in their heads? I think there's some cognitive dissonance going on... 
 As a foundation text for modern paganism it often appears in bibliographies and notes to bolster neo-pagan ideas...yet it has little substance itself. Many of them unquestioningly use TWG as a source and assume an authenticity and robustness to Graves' arguments that just isn't there...like building in a bog without making sure the pilings are solid. 
 He has become an authority figure and this text is used again and again when discussing ideas around goddessy and sacred king type ideas, yet on examination many areas he explores in TWG are his own with little basis in ancient tradition. I wonder if many folk that have read this have actually paid attention, I mean critically read it? Or if most just assume Graves is correct and then use him as ref? 
 He seems as enthusiastic about ritual killings and orgies as he is about the whole tree/calendar/alphabet thing...yet I've never known anyone that has read this book wax as lyrically on those subjects. 
 And the drivel he spouts on lame kings is interesting but still drivel... 
 But I can get Graves' text as a personal mythology and as something to give insight into his other work...and I get the fact that he had no intention of starting a pagan revival (see his disdain for wicca and neo-paganism in general in some of his work). But, for me, it is pretty much all supposition based on guesswork based on reworking of bad translations. 
 Now I shall put this away and never touch it again...after I have driven a stake through the text...the stake obviously of a wood that has suitably mythopoeic resonances for true poets.