Well... yeah, we've used it successfully. It's not quite designed for super-high fantasy (D&D style), but works far better for 'low-magic' campaigns. A couple differences, some of which we worked on (and others not 'cause it wasn't important at the time in order to play anyway)..
*Basic Classes will have varying amounts of 'inappropriate' class skills and bonus feats. If you give none of them anything, then some will lose more out of their base lists of skills/feats than others. We decided temporarily that this is okay, and up for debate later.
*Craft skills need some kind of tweaking. Every weapon and armor would fall under the same skill, and the Craft times are extremely short for, say, making a suit of full mail. Additionally, and most obviously, some of them are unfitting.
*Many of the feats and skills simply don't apply at all. This has one other effect. By default, there are no Proficiencies for the "Rogue's list" or "Druid weapon proficiencies", or what have you. Unless you deem it worthy of fixing (which it may not be) and take the time to do so, there'll be two kinds of people. Those with Simple Weapon proficiency, and those with Proficiency in Every Nonexotic Weapon In The World.
*Armor, on the other hand, will generally take several levels to become fully proficient in (Assuming no changes to the standard system), and you'll see many more people start to question the real value of heavy armor as the rules go (I have to spend three of my character's feats to learn this, it slows me down, reduces my skills, and boosts my armor by two? Nah.)
*You'll have to decide if you like d20M's range increments for weapons and cut down the D&D ones to about a third of their listed values, or decide if you don't mind leaving them how D&D has 'em (Well, since you're *Tossing* the books, you might wanna jot some of those other weapons down on paper or something.. d20M only has modern compound bows and crossbows).
*Many of the Advanced Classes will need changing. For some of these, it's pretty easy (Soldiers make perfectly acceptable fighters with a few skill and bonus feat changes). For others, it's .. less easy. If you like Mastercraft objects, you'll probably need to figure out a way to Build a Better Techie.. one more suited to a Medieval world, anyway.
*You'll have to do a lot of Advanced and Prestige Class writing yourself. Some concepts from D&D become a bit harder. Take D&D's new "Marshal" class from the Miniatures Handbook excerpt. His knack for leading affects a lot of people. In d20M, I haven't found *any* knack for leading that affects that many people. 1-5 people (A party, go figure) is generally the limit, with a few exceptions (and the benefits are considerably less dramatic, too).
*Stuff I forgot to mention goes here.
