Another for the 'what were they smoking' category, the filcher has a number of... interesting design choices. A single foot would be weird for an ethereal creature, let alone for a material one. If the thing on its torso is its face, then what's that wormlike thing atop its body? Why is an underground creature colored brightly green?
Their appearance aside: filchers aren't exactly strong. The 5 aberration HD suck and there's only three (middling) skills to spend points on. The dexterity bonus is huge, but the intelligence penalty is especially unpleasant on what will probably end up being a skillmonkey. The bite attack is weak, though it shouldn't be hard to convince the DM that filchers can wield weapons, opening up several interesting tactics.
The skill bonuses (+8 to SoH, +4 to Listen and Spot) are pretty large, and Detect Magic at-will has its uses (though of course, a mere 2nd-level warlock can do that and much more). I guess the chief reason for playing a filcher will be Ethereal Jaunt, which turns you ethereal for a round at the time with nearly no actions lost.
Can a viable build be made here? I think the answer is yes. The ability to be ethereal whenever it's not your turn is pretty huge, and the other abilities have at least some use. Still, I think +0 LA is enough.
Next are Ethereal Marauders!