It brings some new content to Heroes III, enough to make it a must have for fans of the game.
The new campaigns are set after the events of Heroes III, the first campaign in which the expansion is named after focuses around the "Armageddon's Blade", and the rest of the campaigns are not really at all related to the original Heroes III campaign, or Armageddon's Blade. Despite this they are solid stories, and have well designed missions.
Armageddon's Blade also features a new town called "Conflux" which focuses around Elemental heroes and creatures. You can finally recruit your own Fire, Water, Air, and Earth Elementals from the Conflux, and it's a very welcome addition to Heroes III.
Along with some new scenarios to play through, Armageddon's Blade adds a random map generator to the map editor, and quick RMG for use in easily generating random maps from within the game itself. The RMG in AB is an improvement over the ones present in the first two games, and will give Heroes III fans infinite replayability.
Armageddon's Blade is worth tracking down for fans of Heroes III who want the Conflux town, and random map generator. The new campaigns and scenarios are entertaining of course, but the former two features are the primary reason this expansion is great in my opinion.