Well you do still get the HoT effect on any Flash of Light when you have SS up but ya, other than that it's damn near worthless for a mere 2k~ dmg absorbed in a 30 second period. This is going to really hurt ret survivability in PVP since most good ret players ALWAYS have this up.
Also I wouldn't say it's terribly hard to use, but it does require some skill. You have to watch and make sure the spell is still up and also watch for the actual adsorption effect to go off if you want the increased 50% crit rate on your FoL.