"The yuan-ti are descended from humans whose bloodlines have been mingled with those of snakes."
Yeah, I don't really want to know how that works. Experimental druids?
Anyways, yuan-ti. They have relatively little in common, not even their body type, but do all share the ability to Detect Poison at-will (a wand is what, 400 GP?), and turn into vipers ranging from Tiny to Large size (hello there, free poison!).
Also, yuan-ti enjoy a smattering of exclusive game material, including a category of grafts, a subpar arcane casting class, and a divine casting class that would honestly be acceptable if it didn't make you eat so many RHD.
4 RHD monstrous humanoids with mildly increased stats (+2 dexterity, intelligence, and charisma). They have Alertness and Blind-Fight as bonus feats (meh), get a +1 bonus to natural armor, gain SR equal to class levels + 14, and can cast a number of low-level spells once per day. All in all, I'd call them a bit underwhelming, and just barely undeserving of +0 LA. -0 it is!
Seven monstrous humanoid HD this time, but a few additional abilities. Ability scores got increased across the board, especially mental ones. Halfbloods also have a poisonous bite attack that delivers constitution-damaging poison (not bad), the ability to add acid damage to future natural attacks (crap, as it's a standard action), a major bonus on Hide checks, and SLAs that are still underwhelming (Entangle is probably the best of the bunch). SR is 16, making it actually worse than what a pureblood of equal ECL would have.
A few alternative halfbloods exist. Variant one gives up opposable thumbs and its bite attack for two new, weaker bite attacks, which is awful for obvious reasons. The second one is basically a free swim speed and the ability to constrict Small creatures, so there's no reason to not take it (not like it alters LA, but still), and the third one is the second with some land speed exchanged for climb speed and a slight buff to the constrict ability (probably not worth it).
Honestly, I'm kind of disappointed by this monster. Mental scores are excellent, but it has only one useful SLA with a saving throw. Its other features seem to encourage grappling, except this thing has no Improved Grab, is medium-sized, and is no stronger than your average orc. The only remaining route is dealing dumb melee damage, which I can't recommend in good faith. -0 LA.
9 RHD this time, and large size. Ability scores are better, though still somewhat lacking. At least these have Improved Grab.
Chief of their magical abilities is Aversion, which makes people afraid of snakes and basically forces them to keep 20 ft. of distance at all times. It's a pretty potent debuff against melee brutes with low will saves, but then again so is a ton of other stuff by now.
Constrict is okay, Produce Acid is still bad, the SLA repetoire got slightly more uses and has been expanded with 1/day Baleful Polymorph (would've been nice if the party casters weren't getting it by now as well). Their version Baleful Polymorph can only be used to turn people into snakes, which is admittedly kind of amusing when combined with Aversion.
I've said it before and I'll say it again, these guys are underwhelming. For their level and status, I'd expect at least some higher-level SLAs, or perhaps better strength. As they are now, I wouldn't recommend them. -0 LA.