There are a couple of other links that will be relevant: Adam Roberts' original review on Strange Horizons, and another interesting discussion of the book by Andries du Toit.
Rather than respond on the individual blogs, I've chosen to write my response up here. I'm not going to print my review here, though inevitably my response to the discussion will end up covering some of the same ground. (But I've left aside, largely, my comments about Lavinia being a feminist novel, in giving a voice to a character pretty much treated in Virgil - if not so much in Livy - as a piece of meat.)
The problem with coming late to a book that has garnered almost universal praise is that one's expectations are set very high, so high, in fact, that usually the actuality cannot possibly meet them. As with Geoff Ryman's Air I was prepared for a transformative book, and as with Air, I was slightly disappointed when it turned out merely to be very good. (And that then leaves me wondering if the problem isn't simply that I'm too dumb to pick up on why the novel is so great ...)
Lavinia has moments of brilliance. I love the function of the shield (something taken directly out of Virgil), and the way different people see different things in this, frankly impossible, object. I like the way Le Guin creates a Bronze Age Latium that has yet to fully anthropomorphize its gods, and how that interacts with the arrival of a Trojan culture that does (whether this actually represents how people in the Bronze Age thought about their gods is, I suspect, unknowable). But I agree with Abigail Nussbaum that the last third of the book isn't anything like as effective as the earlier bit. The earlier sections have a complex structure, akin to what Adam identifies as one of the strengths of the Aeneid. But then it becomes, as Abigail says, rather a straight narrative: 'this happened, then this happened ...'
There's some discussion about whether one needs to have read the Aeneid to appreciate Lavinia, and what is the effect if you haven't. Certainly, it's inescapable that this is a novel in dialogue with the Aeneid (or as Cheryl Morgan puts it, it's 'Virgil fanfic'), just as much as (so John Clute tells us) Greg Bear's City at the End of Time is in dialogue with William Hope Hodgson's The Night Land, or Stephen Baxter's The Time Ships is with Wells' The Time Machine. So yes, if one is not familiar with the Aeneid, part of the conversation will be missed. What interests me is that, where the source text lies within the sf canon, this is seen as less problematic than when the book lies outside science fiction. (Also, everybody should read the Aeneid anyway.) And one of the things I like about Lavinia is that Le Guin chooses to have that dialogue with the second part of the epic, rather than the better-known first half (escape from Troy, Dido, etc.).
Abigail comments that she finds the existential moments a bit heavy-handed. For me, those moments are one of the points of the novel. At its best, when Lavinia meets the spirit of Virgil in some timeless zone, this work seems to me to be a musing on the nature of fiction. Once our lives are written about, are they any longer our own? What is the nature of the relationship between the 'real person' and the person that exists in stories? Le Guin raises these questions, but doesn't answer them. Can anyone? It's this postmodernist metafiction that intrigues me most about the book. This doesn't get much coverage in the discussion, with Niall even suggesting that he thought Virgil could be removed. I think Virgil is what the novel is about (and du Toit seems to follow this line of thinking as well).
Is Lavinia a fantasy novel? Perhaps another question that can't be answered. What I note, however, is that it does not read like a typical heroic fantasy novel, translated into a Graeco-Roman (semi-)mythological context. There have been quite a few of these recently, such as David Gemmell's Troy trilogy, or Jo Graham's Black Ships. These are both, for the most part, historical novels, yet written in the idiom familiar from the quasi-mediaeval fantasy. This is not necessarily to knock these books - Gemmell's is quite an interesting treatment, Graham's less so. But Lavinia doesn't follow that road - in this it is perhaps more like Gene Wolfe's Soldier series, which works in a fantasy mode that owes little to the traditions of north-west Europe.
Cheryl makes an interesting comment about the gender roles portrayed, and the apparent hostility to gay men. There are several points to be made here: (1) To think of things in terms of 'gay men' is probably to impose twenty-first century categories on Bronze Age sexual behaviour; (2) a writer should never be held to the opinions of their characters; (3) Lavinia comes from a time when gender roles were different from how they are now, and in some ways quite rigid, and one shouldn't expect Le Guin to write her with more modern attitudes. That said, Le Guin's portrayal of Ascanius as a man who behaves in an unseemly fashion with his male lover, and grieves excessively after his death, whilst reflecting Roman attitudes to this sort of behaviour, does seem oddly hostile, and doesn't seem to be based on anything drawn from ancient sources (though I'm prepared to be proved wrong on this). I think Le Guin is turning on its head Roman legend that portrayed Lavinia as, in her later years, an evil crone that worked against her stepson, and is seeking a motivation for Ascanius.
All this discussion has made me think again about this novel, and want to reread it. Sadly, there are other things I must do.