Improved Spell Resistance

Removed in Update 8
As of Update 8 the Drow Spell Resistance line of enhancements has been removed. Drow now inherently possess Spell Resistance equal to 10 + Character Level.

Hard nerf in 3.3 update
This enhancement line used to be way more effective than as is, but got nerfed pretty hard in 3.3 enhancement revamp. Nowadays it is barely taken, as SR 20 for 10 action points doesn't worth it at all, compared to SR 19 random loot (belt or ring slot), SR 22 raid loot (Ring of Thelis) or cleric Spell Resistance buff (caster level + 12 SR).