its not isolation like a preacher curl...but it fries the hell out of them. just ask bfold how many curls he does and he'll say...i flip tires. overload those biceps either with direct work or hard indirect work, you'll get results. i think the carryover will defitely help your other lifts by working your back too. thats the plus side to it. preachers are alright, but theres more than one way to skin the proverbial cat.
how cryptic was that? hehehe.