The Underdark warriors seem like they're just general "boost" classes... Some nice bonus feats and skill boosts, but nothing really unique. I pick the specialist characters that have unique benefits, or at least really cool ones.
Take the Twilight Huntsman, for instance. At the seventh Huntsman level, they are given the ability to "phase" in and out of the ethereal realm at will. I'm never going to get high enough in levels to get that, but it's still damned awesome.
Alienists have some really awesome abilities, but they're bugged to hell at the moment so that's a moot point.
Shadowdancers get access to shadow "pets", along with an innate ability to shadow step at will.
Blackguards are basically anti-paladins, and get several paladin-esque abilities that are tweaked to better suit the "dark side".
Earthsingers get some seriously cool earth-related abilities, but their requirements are too high for the current level cap. You can't get the required skills before you're too high of a level to access the earthsinger's special goodies.
The tattoo mystic, of course, gets some nice tattoos. Life-leeching, fire-blasting, brachiation-providing, poison-inducing tattoos.
The assassin gets death-effect sneak attacks. Damn straight.
So I'd rather waste my time going for one of those than waste my time going for something else. Xavias-worshiping loremasters can be fun, but that's mostly just for making sunrods with alchemy and then feeling really good about it.