It's basically mostly useful to players of Martial characters. Although other player's who multiclass might find it useful too, or DM's that want to use PC type villains.
It's a very focused book, there is a little bit of rolelplay fluff here and there but it is mainly powers, feats and features. No new equipment, the only monsters as such are animal companions for the Ranger and they can't really be used on there own.
Really just started scanning through it, but the fact it is packed with so many new powers impressed me. Fighters get four new At-Wills, (not including utility one's at higher levels like the Rogue got in the PHB), Rangers and Warlord's two and the Rogue just the one. There are six new 1st level Encounter Powers for Fighters and five new Dailys, each level (where you get powers) seems to get at least four or five. Then 12 new Paragon paths for Fighters. The Tempest Technique or Battlerager Vigor Class Feature makes a lightly armoured fighters a real possibility.
You get a similar number of extra powers and paragon paths for the other classes. In the Martial Option section there are two Martial feats open to any martial class then between 15 and 20 feats for each of the martial classes in each of the heroic and paragon tiers and more general martial feats and a handful or extra class specific feats in the Epic tier. So loads of new feats. (edit)
One thing that does concern me it he Battlerager Vigor means it could effectively have unlimited temporary hit points, since it's Feature allows it to stack temporary hit points gained from Invigorating (new keyword) powers with any other temporary hit points they already have.