Q: How does one make the soul-sucking undead creepier?
A: Have the soul-sucking undead's previous victim be imprisoned and visible in its torso.
Let's get the bad news out of the way first: devourers have 12 undead RHD. That means missing out on 6 BAB if you're a warrior-type, 6 initiator levels if you're a martial adept, or 12 caster levels if you choose to be a caster (yeah, that'll happen).
Their stats aren't great either. No constitution is annoying, dexterity is no better than a human's, and the mental stats are moderate. Strength is pretty high, but if one wants strength there's better things available.
Two shortsword-strength claws are pathetically weak for this level, and I don't recommend ever using them (no, not even if your DM disallows wielding weapons in them). Even the free Energy Drain isn't worth it: Soul Eater nets you the same for eleven less sacrificed levels.
The signature ability of devourers is Trap Essence: with a successful attack roll followed by a failed saving throw, any creature can be permanently imprisoned in their bodies. That's pretty great, because very little short of avoiding the attack or making the save defends against it: one might even argue that because the text refers to 'essence' rather than 'soul', the ability works even against soulless creatures.
Devourers with a trapped soul get two more abilities.
The first basically allows them to cast SLA's (ranging from the outclassed Ghoul Touch and Confusion to the useful Control Undead and Suggestion to the great Lesser Planar Ally and True Seeing) by siphoning their victim's soul, which is simply great.
The second lets them redirect a number of spells to the trapped soul. This is a strange ability as you're immune to most of these spells anyway (Detect Thoughts? Suggestion? Dominate Person?). However, it also provides protection against several SoD's, so be glad you have it. People trying to cast Magic Jar on you are especially screwed.
So what LA to give here? Offensively Devourers can kill one being a round: the lack of immunities against the attack balanced by the fact that it requires an attack roll. That's strong, but not impossible at these levels (compare to the Sorcerer with Versatile Spellcasting casting six Disintegrates or Circle of Deaths a day). The SLA's add some extra tactical options, but not enough to justify raising the ECL beyond 12 IMO.
In the end, I'm going to assign a LA of +0. The reasonably strong base abilities are balanced by the fact that there's very little room for advancement in any direction.