Does anyone play this? Is it any good? Is there a guide?
After reviewing the class, I wonder how useful is it.
It's really not that good of a prestige class, unless you're in a party where nobody has any points invested in Knowledge (Anything), and you have no bards. It's skill prerequisites require at least a level 7 character which should be enough time to tell the people at your table that not having knowledge skills will lead to them not knowing anything about the things they fight. The prerequisites of a three metamagic/item creation feats pretty much leaves this as a prestige class for someone who wants to act as the party's quartermaster/supplier/textbook. A pure support role that will only be fun for people who are specifically interested in being the guy who can get/tell you what you need.
In short, only ever in a 6-man party where everything else is covered.
For most primary arcane spellcasters, the only real requirement is investing in 7 different Divination spells. As a primary caster, you are most likely going to be taking at least 3 metamagic feats, so that's no big deal, and Skill Focus in Knowledge isn't too uncommon, and most spellcasters will keep at least Kn:Arcana or Kn:Religion capped out, as well as Kn:Planes.
As a Wizard, you won't be losing much by taking the PrC. You'll lose your 2 free spells added to your spellbook at each level, but you can still find scrolls and copy out of spellbooks. In return, you basically get a few extra feats. If you wait until 8th level, you won't even miss out on school abilities. Loremaster isn't too bad for a Wizard, as long as you have the cash to shell out for spells.
As a Sorcerer, the requirement for knowing seven different divination spells is a bit more harsh, and you don't have as many skill points per level, so keeping two knowledge skills capped is more difficult. You also lose out on progressing your bloodline abilities. I'd avoid Loremaster as a sorcerer.
As a Cleric, the Divination spells requirement is a non-issue, but you might run low on skill points. You loose out on BAB and Channeling dice. You gain on other class features, and if you wait until 8th, you'll get your second domain abilities to boot. I'd recommend it for a casting focused cleric.