I tried kiting a magtheridon channeler when we were about to wipe, and slow doesn't work on them. It may increase cast time (couldnt tell because there was always COT up) but it definitely doesn't decrease their movement speed.
I know that slow would be too powerful if it worked on bosses and all kinds of mobs, it would make mages mandatory for most fights, and it would also pigeonhole all mages into 41 arcane. But I'm annoyed that the spell often gets applied, takes up a debuff slot, but doesn't actually do anything. Just make it immune then, but no half assed stuff...
Anyway, outside of raiding it's still a very good spell, I soloed all kinds of elite mobs like the defenders in heroic SP, the felguards in heroic BF, very useful there. In pvp it's kind of a mixed bag. It's a 30 yard, ranged, instant, spammable snare with no diminishing returns, which can be cast on the move and without facing the target. It's rather underrated. It's awesome for stopping flag runners in WSG. Most druids will run oom from shapeshifting before you do. It absolutely destroys shamans and hunters, because it also halves their ranged attack speed. Shamans can't remove it and kiting them becomes trivial. It's good against warriors. You can deny their charge and also snare them instantly. Trinket? Just reapply it. Rogues? It doesn't help against good ones, but dumb ones who don't shiv crippling or don't open with garotte can be kited easily. Most will pop cos when you apply it, just blink, run away and reapply it.
Against mages? It screws fire mages hard, but doesn't do much against frosties because they can kill you with just using instants. Same thing with warlocks. Screws destro ones but doesn't do a damn thing against affliction or demo. Against priests, pallies or druids it's pretty useless.
The most fun application for me is still the slow-sheep combo against horde trying to ride past IB >

Dismount them, let others deal with them and proceed to ez-mode dismount the next one!