Fire/Ice Wall IS useful.
while the wall is up: it blocks all debuffs from warlock (& chinese force too i think)
No kd or kb while wall is up
Fire wall can take 1 nuke, giving u time to select & kill the wiz
Ice wall is very usefull as int when glavier just critted u
Fire Wall against a blader is very usefull as they have little mag att so it stays on long
2 Limitations:
U can NOT move in the wall.
A skill that successfully stuns u, removes wall instantanous. (not the warlock stun, it's debuff)
U have to use the skill again to cancel the wall (or right click on it; next to ur char name

)
It's worth having this skill maxed @ high lvl