This is one of those perennial threads that pop up every few months. Let me see if I can speed this one up a bit.
First thing to get out of the way: the Minis Handbook is 3.5e not 3e. Somebody always comes along and says it's 3e, and it ain't.
Second, the base classes: of the four basic classes presented, the Favored Soul and Warmage have been presented in other books. The Marshal and Healer have not, and you won't know if they're any good just by asking people because opinions vary widely on both from "overkill" to "pathetic".
After that, you also get a smattering of prestige classes, including the insanely powerful War Hulk. The book has a few pages of feats and magic items you haven't seen elsewhere, and some of them are pretty cool.
The spells are good too, but they've been repackaged and updated in the Spell Compendium.
There's a monster section, most notable for introducing aspects.
All of the rest is worthless, obsolete minis rules.
It's an OK book, but not worth paying full price for. Get it for cheap on eBay.