Achiever did help me improve my scores significantly when I retook my OAT. It has challenging problems that often made me work a lot harder from there. As far as I know, OAT Achiever is the only computerized sample tests that provide you with detailed explanation.
yeah dude, the RC section on topscore is no where difficult in terms of preparing you for the RC on the real OAT. It has no solutions, just answers. I never tried Achiever, but i have tried topscore and i def dont recommend it.