My 60 necro has never been able to land a slow on one, but the message says it's resisted, not that the creature can't be slowed. These things seem to be pretty magic-resistant, and they can't be hit by spells unless you're in melee range.
I can land spells on these fairly easily.. the trick is that like dragons and giants, you must stand right under them (the closer to target ring the better) or No spell will land.
Had a Breastplate of the Righteous drop off one of these guys. Nice piece and assume it must be rare. Luckily had a Pally in the group. He was happy ;)
When my guild get's into PoH and start the raiding/slaughter or NPCs these guys are usually right in the way. I don't mind these guys now that I know about the Marrsinger gems