I really love the fact that Shellder can KILL Abra everytime as other Smashers will be devastated by it with Focus Sash Intact; maybe Dwebble can do the same but Rock Blast' accuracy may leave it open to revenge kill.
I'm really thorn between Clamperl and Shellder. It just depends on the team you're running. I find it easier to fit Shellder on my teams but Clamperl has proved that it is really frightening with a DeepSeaTooth.
I used Omanyte on Sand and it REALLY is powerful having good Def and good SpD even after a Shell Smash. I don't know how it would fit on a standard team. I mean, it's really good but Clamperl is better IMO.