They're really a glass hammer and hard to use, although in the right situations they are awesome in a pinch. Think of them as Vypers that can deepstrike.
Can't Vypers deep strike already though? They are skimmers after all
My personal experience with shadow spectres has been pretty positive so far. I usually field them with a Pale Courts Battlehost formation that has a Dire Avengers core, so they get used almost exclusively for vehicle hunting duty (since Dire Avenger rifles can handle anything with wounds given the bladestorm rule).
They usually fly around beside or behind my dire avenger squads, providing haywire or prism support and I never deep strike them. I don't recommend deep striking them due to their low body count and not being able to take advantage of their cover save the turn they come in (they need to thrust in the movement phase to gain the cover save).
Given this setup, they combine to make a really powerful one-two punch in situations were you need to take down a transport, then gun down the passengers in a single turn. More than once I've had opponents try shoving land raiders packed with a deathstar unit down my throat, only to have the land raider get wrecked by the shadow spectres then lose their super expensive squad to massed shuriken fire. Feels good man.
One thing I'm unsure about is how well shadow spectres do as more than one specialized unit included in an army. Playing an army built around a core of shadow spectres might be an interesting choice, given you have the models of course.