I gave it a 10. Not because it's incredibly broken, but because there's simply no downside to the class for the people it was intended for.
We've had this discussion in other threads, I'm sure someone can find them. Basically, I'm one of the people who doesn't like the "+1 spellcasting level at every level" classes. PrCs, IMO, should be straightforward to get into, with prerequisites being things you'd already want to take, but where the class itself has drawbacks compared to your original class.
Once you qualify for Loremaster, you get full spellcasting progression. Your class skill list is better than what you had before (including all Knowledge skills AND Speak Language), you get 4+INT skill points per level, and some nice extra bonuses. The class gives two extra languages (yawn), a toned-down Bardic Lore ability, Identify/Legend Lore at will, and five Secrets. Most Secrets are conservatively the equivalent of half a Feat, except of course for the "Free Feat" one, so figure that you gain three Feats out of the deal.
So, the only balance factors are the prerequisites.
For a Wizard, you'll get two metamagic/item creation Feats at levels 1 and 5. That means if you use two of your three (or four for Humans) general Feats for another metamagic and Skill Focus, you've met the Feat requirements.
The Knowledge skill requirement is easy.
On the other hand, you sacrifice two extra bonus Feats in the 10 levels of the class, so you're trading one general and two bonus Feats for the benefits of the class (which includes three Feat-equivalents and more).
For Cleric/Druid/Sorcerer, it's a lot harder to get in (since meeting the Knowledge skill requirement means level 17), but it becomes pretty easy if you multiclass. A Cleric X/Bard 1, for example, can now meet the Knowledge requirement. Or, use Aristocrat. Or, add the Educated or Cosmopolitan Feat from the Forgotten Realms.
I've played a Human Sorcerer Loremaster before. The most painful part was actually spending 20 skill points to raise the Knowledge skills; Sorcerers don't get that many since they don't need INT. Spending 30 for cross-class on one would be out of the question.
Bards are somewhere in the middle. They can get the Knowledge skills easily, but the Feats hurt a bit and their spell progression is lower. They also don't gain as much since their Bardic Lore is already better than what a Loremaster gives.
Bottom line: it's really only intended for one class, and for that class there's no real downside. Its flavor implies that it's meant for Diviners, but an Evoker can take the class and end up a stronger character.
From a powergaming point of view it's a great class. For me, it's exactly how I think PrCs SHOULDN'T be designed.