Urban fey are always interesting to see, and I honestly like this monster quite a bit. Essentially, they're insane spirits that live in piles of disused lumber, kidnap children, transform them into shrubs, and eat them.
How do those charming creatures stack up as PCs? Surprisingly well. Two fey RHD sting a little, but aren't that much worse than two rogue RHD. The stat boosts are surprisingly large: +10 dexterity, +8 intelligence, +6 charisma, +4 wisdom, and +2 constitution. That's a net +30 to ability scores, for two RHD.
Furthermore, splinterwaifs (splinterwaives?) have DR 5/silver (curiously enough, that's silver and not cold iron). Their base land and climb speeds of 50 ft. are blazingly fast, they get Weapon Finesse and Improved Initiative for free, and on top of all that splinterwaifs get 2d6 sneak attack. They also have some natural armor (+2), SR 7, and a racial +10 hide bonus.
Weapon-wise, splinterwaifs have a natural claw (1d4 damage) and the ability to spit a thin wooden splinter (1d6 damage) with what's otherwise a fairly unremarkable natural ranged weapon.
The splinterwaif's special abilities are worth mentioning too. Call Bramble lets it create a bramble with the stats of a medium animate object as a standard action: it's not great, but it might be worth using at low levels sometime.
Camouflage let's the splinterwaif give itself +10 to Hide and concealment as a full-round action, until it moves. It's obviously better on an ambusher monster than a PC, but even so it's not useless.
Superior Woodland Stride is exactly what you'd expect: the ability to travel through dense undergrowth, brambles, thorns, and what else. It notably also grants immunity against magical versions of those effects, meaning that stuff like Entangle isn't an issue anymore.
Finally there's Transformation, which lets the splinterwaif turn dead fey and humanoids into bramble bushes. It's mostly a fluff ability, though it does have some utility in preventing Raise Dead and Resurrection from working.
What LA is appropriate here? I think we can all agree +0 is far too low, and in my opinion +1 is too. Comparing the splinterwaif to a human rogue or swordsage, at +1 it has somewhat less HP, much better stats, Damage Reduction, the same amount of sneak attack, and just as many feats (though the human has more flexibility in which ones to pick).
Honestly, I feel like +2 is the proper LA here: it strikes a good balance between 'too fragile to function' and 'hyperfast murderthorn'. +2 for now, looking forward to feedback.
Four more RHD haven't done the splinterwaif a lot of good. Its sneak attack has gone up by +2d6, and its spell resistance is a bit higher, but its stats are otherwise unchanged. I think this is still viable at +0 LA, though.