Andy what's your take on incline curls. tried them before?
If you look at the barbell curl you will note that there is an 160 degree of rotation.
During the first 90 degrees of movement in a curl, for example, the direction of movement is constantly changing from horizontal to vertical, and the weight will thus seem to get heavier – but after 90 degrees of movement, the direction of movement starts changing from vertical to horizontal, and the weight will seem to grow lighter.
It's interesting to observe that the so called sticking point is where the arm is horizontal, there is very little or sometimes no resistance to the muscle in the contracted position or stretched position, which seem to be the most important areas for improved muscle strength and flexibility.
Changing the angle of the torso also changes the time that the muscle is placed under load, but because of the nature of the barbell you will always encounter changes in resistance throughout the movement.
If you look at the biceps you will note that the sole purpose of these muscles is to suppinate the hand, to prove my point; try flexing the bicep while your hand is in the "goose-neck" position, now suppinate the hand.
The other function of the bicep is to lift the arm up over the shoulder.
We know that the best exercise for muscle growth is where an exercise places the most resistance in the contracted and the stretched position of the muscle.
As far as I'm aware there is no such exercise for the bicep, for to achieve this you would need to lift the arm up over the shoulder under resistance.
To illustrate my point; clench your fist,, suppinate your fist, and flex your bicep, now lift your arm up over your head, now push down as if you are trying to scratch your back, and hold it there, flexed as long as you can.
Your biceps will cramp when you release and you will feel the effects of that for some time after, because it's rarely under load in that position.
Some believe the the preacher curl is an effective exercise, it would be more effective if the arm was positioned higher and instead of using a barbell you would use a cable.
I would say that if you where really interested in building muscular size of the bicep, then using a variety of differing exercise, would be a good start.