I bought Noble Steeds at the same time I picked up Mutants & Masterminds, so NS has gotten only a cursory readover.
Noble Steeds has lots of nice real-life info on horses, some interesting ideas about giving horses classes of a different sort (e.g., they don't get BAB, saves, HD, etc., every level; the owner has to dedicate XP to them to increase their level), some cool steed feats, and a few typos (I think the light and heavy horse stats are switched; at the very least they're different from the MM's, and are very odd -- IIRC, the light horse has more hp than the heavy). I don't recall whether there was specific info on integrating their steed classes with things like paladin's mounts & the like; I don't think there was, which is a real shame.
It also has one of the most unintentionally amusing sidebars I've ever seen; there's a little rant about how stupid and unrealistic riding dogs are, how they'd never work, etc. It's on the page where the section about "legendary steeds" starts -- you know, things like horses with wings, lion/bird crossbreeds, magic horned horses that only like pure & innocent virgins, and other highly realistic stuff like that. The layout person managed some great ironic comedy, intentionally or otherwise.
As I said, I've only skimmed it, so any conclusion is preliminary at best; but my impression was that it had some cool real world info, some neat ideas, unfortunately flawed execution (rules-wise), and silly pointless ranting.
