I think it's really, really good.
However, it's one of those books that is only worthwhile if you play both the skirmish game and the D&D RPG. The mass combat rules are adequate, and fine for battles of up to about 250 troops a side. Well, maybe more - 500-1000 a side should work fine... though I haven't used them yet.
However, the Mass combat rules don't seem to scale up to Helm's Deep size very well. (Which is why I've ordered Cry Havoc).
Returning to the sections they did well: the new D&D material is excellent. The new classes are interesting, as are the new feats, spells, magic items and monsters.
The Skirmish rules expand the basic game in a number of interesting ways. Rules for new terrain types are explained, there are dozens of scenarios, and their are rather fun campaign rules (including magic items).
The conversion guidelines to help you convert D&D characters and monsters to the skirmish game are adequate. Actually, I think they are about as good as you were going to get. People obsessed with formulae aren't going to like them, but there was no possible way such could be created for the game.
Cheers!