I find it completely useless. Enemies with shields aren't as plentiful as the were in Borderlands1, and those that have them lose em quickly. Shock against the player works well for the enemies as it negates your own shield in a second though. I personally don't even carry weapons with shock.
Yep shock is completely useless in my opinion as well and even in the first game I hated seeing shock. Certainly seems like with BL2 as it is now caustic is the most powerful. Of course BL2 hasn't received any of it's addons or DLC yet.
I like to carry Shock weapons because not many enemies seem to be resistant to it (only one I remember are ION Loaders and Shock skags, probably more), and it served me well on TVH-mode with Zer0, especially against Stalkers. They're pretty good as an all-around element, especially on grenades.
But on the other hand, while there are plenty of shielded enemies in TVH, I feel normal damage and slag transfusion grenades can produce the exact same effect, if not better... Still, Shock ain't useless. I don't know if it's the worst element of the bunch, but it's not useless.