I've read the first two books (the only ones that are, at this time, available) and I think a few things should be mentioned.
For one thing, while I've heard it said that all adventure paths are railroads, that's especially true for this one. As it's laid out, you begin the campaign with very little freedom (which gradually increases as the campaign goes on), as the PCs' patron basically strong-arms them into working for him. Saying no can very well result in a TPK.
Can the GM modify this? Sure, but there's comparatively little leeway in doing so, unless major changes are made. The first part of the first adventure goes out of its way to isolate the PCs in a hostile environment, so as to give them little choice but to agree.
Likewise, the campaign flat-out says that no good or chaotic characters are allowed. Anyone who wants to play a chaotic evil barbarian should be told flat-out that such a character isn't appropriate for the campaign.