Right, what he said.
Many PrCs are "overpowered" during their 5 or 10 levels, being absolutely better than the class they build off of, but this is compensated for by increased requirements (which usually end up requiring 5 or 6 levels to acquire). I hate these classes. For one thing, you can often acquire several of them, since prerequisites often overlap. (Once, I made an NPC opponent who mixed ShadowDancer, Blackguard, and Assassin.) But also, it means that you're making a weak character for several levels in exchange for big power later on, while I prefer a more smooth progression.
But the other type of PrC, the one I vastly prefer, is one which has some sort of ongoing cost. Usually, this is by dropping or reducing some progressive part of the core class (spellcasting, familiar/companion/mount progression, skill points, hit points) in exchange for something of comparable value.
The Horizon Walker is one of my favorites of these; yes, it requires 8 ranks of a Knowledge skill, but that could be reduced easily without breaking the class; it really only serves to limit the class to Rangers and Bards. Its benefits are pretty much equal to what a Ranger would give up to take the class; the -2 skill points per level are balanced by the skill boosts most terrains give, the loss of Favored Enemy is balanced by the attack bonuses some terrains give, and the loss of spellcasting is nicely balanced by the innate abilities some terrains give. It all works nicely.
These PrCs are not better than the core class, just different. For this sort of class, having very weak requirements is just fine; you're effectively creating an alternate core subclass that's well-balanced with the existing material.
