The Miniatures Handbook is a grab-bag of a lot of different bits of information. Is it a good book? Absolutely. It has some of the most interesting designs of the range. However, they tend to push at the limits of game balance - if only because we've never seen anything like them before.
Here's a brief run-down of the D&D material (68 pages)
* Four new base classes: Favoured Soul, Marshal, Healer, Warmage
* Seven new prestige classes: Bonded Summoner, Dragon Samurai, Havoc Mage, Skullclan Hunter, Tactical Soldier, War Hulk, Warchief
* 22 new general feats
* 10 new metamagic feats
* The Swift action type (Immediate was added in the XPH)
* About 70 new spells
* New Magic Items
* About 30 new monsters
Here's a brief run-down of the Miniatures material
* Explanation of stat-cards and converting D&D stats to D&D Mini stats - 10 pages
* Full skirmish rules, including scenarios, campaign rules, and terrain rules - 44 pages. (Although the actual skirmish rules have been superceded, the scenarios, campaign and terrain rules have not been reprinted, so half the chapter is still useful).
* Mass battle rules - uses a 1:1 scale - 36 pages
* Random dungeon rules (basically a way of using the miniatures to play a board game) - 11 pages
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The RPG game elements that I have found most useful:
* The Favoured Soul: although reprinted in Complete Divine, this spontaneous-casting alternative to the cleric fills a great niche.
* Sudden Metamagic feats: reprinted in Complete Arcane, but provide metamagic usable by standard wizards, esp. at lower levels.
* Revivify spell - a raise dead that doesn't cost a level to the recipient, provided the cleric can get there within one round of death. Not reprinted, and absolutely excellent.
* Aspects - weaker avatars (about CR 10), provide actual manisfestations of the gods and greater demons that you can use! Not reprinted.
Cheers!