Farrell's first fantasy book presents a rich world with well-developed characters, and a few surprising twists in the plot. However, the author's usage of Celtic-like terms and names without actually capturing the Celtic spirit, some naive choices of words and sentences, and a horrendous scene near the end of the book unfortunately make it just above average.
The other reviewers have talked at length about the qualities of the book, so I'll just mention the drawbacks that impressed me most.
The first thing, of course, is the obvious similarity to Tolkien's Lord of the Rings (content, ideas) and Jordan's Eye of the World (style and beginning). In our fantasy-saturated world, however, only very rare jewels manage to escape influences by others, especially at the beginning of their author's career. That is why this point is least important of the drawbacks.
Then comes the Celtic world that everybody else so appraises. It is a good attempt, indeed, but it by no means captures the fey Celtic fantasy spirit. Swarming the book with Celtic-like names and terms (lough for lake, dun for settlement, etc.) and mentioning a few fantasy creatures in passing (the fantasy creatures that do participate in the plot have nothing to do with Celtic fantasy) is not enough to make a book Celtic. It is probably enough to make it "Celtic-like", nothing more. If you are looking for a wonderful book that does capture the Celtic world, you'd best obtain and read R.Feist's Faerie Tale.
Then come the politics. The presentation of intrigue is shallow and naive, and very often I was able to guess who the "bad guy" was chapters before. Being predictable is not something that a book could be proud of. To be truly objective, politics comprise only a small part of the book, and everything else is just as it should be - plot twists, good character building, etc.
Last and most important, the terrible ending scene. Imagine Gollum whispering something into Sam's ear right before Mount doom, then Gollum and Sam together grabbing stones and hitting Frodo on the head. Quite ridiculous, isn't it? This is as close a comparison as I would make without spoiling the book for you. I'd just add that some major characters, who had been built throughout the second half of the book (and had been built incredibly well), do make a similar totally illogical and character-spoiling choice so that the author can highlight yet once again (for the hundredth time) the power of the One Ring... um, excuse me, Stone.
Before judging the book by my review only, I'd advise you to read the other reviews in order to get acknowledged with the advantages of the book (well-detailed world, wonderful setting, incredibly good character-building, etc). This review serves strictly to fill the holes that all the five-star reviews have left as opposed to displacing those reviews. Actually, if I had not read all of them before buying the book, I would probably have lower expectations of it and, logically, my impression on it would be higher. Please do not make a similar mistake.
Last of all, I do not like Amazon's grading style - it does not provide adequate margins to distinguish between different books. On a grade of 1 to 100, Holder of Lighting would get 65 from me. If the book-spoiling last scene was omitted (the plot would not lose much, actually), the points would be close to 80. And only the best of the best would get above 95 (that is 5 stars here) from me.
I'm waiting for the second book in paperback. Hopefully Farrell will have learned from her mistakes. She does have much untapped potential.