Finished the book a couple of days ago..
At first I didn't get into it too much - the action sequences and the "archaeological" scenes dragged a bit IMO, and basically, it just couldn't compare with either of my favorite DRGIII works - the Crucible
trilogy, or Serpents among the Ruins
As the book progressed, though, I saw that this was actually a character piece, rather than an epic-scale TOS adventure. I have to say, that looked upon as such, the novel is quite good, and offers great characterizations for both Sulu and Kirk.
The addition of the (by now) much-discussed spoiler/s only added to my enjoyment.
In short - not an all-time favorite, but a great, deep TOS novel nonetheless.