I don't regret buying the Miniatures Handbook, despite never buying a box of actual D&D minis.
The marshal base class is one of my personal faves, although the other three base classes add very little in my opinion - I consider the healer, warmage and favored soul the least interesting noncore 3.5 base classes aside from the pitiful CW samurai.
The PrCs are interesting but perhaps a bit too tightly focused for most characters. Used in conjunction with either Epic levels or the unfortunately worded Hulking Hurler, the War Hulk becomes problematic, albeit incredibly, utterly limited. Many of the PrCs struggle because they're good for a little while but rapidly fall behind the single-class power curve (like Bonded Summoner).
The feats are okay. Nothing I've used extensively, or seen used.
The spells include some nasty options, but I believe they've been reprinted.
The magic items are mostly designed for the large-scale battlefield, but they include the second most useful magic item I've ever encountered (after the humble
cloak of resistance): the
ring of communication. I can't imagine a high-level wealth-by-level-guidelines party not packing these beautiful little devices.
The monsters are quite cool. Equicephs are both nasty opponents and playable PCs, the Aspects (lesser avatars) would be cool for a Greyhawk game, and several playable oddball monsters also put in an appearance.
Overall, I'd rate it behind Complete Adventurer in terms of usable content for the RPG, on par with Complete Warrior. And, of course, if you ever felt like trying the miniatures rules, it would be much more valuable.