NOTES:  The Preacher Curl can also be done using a curlbar (freeweight bar or fixed machine), in which case you can work both arms at once.  However, I am a big fan of alternating arm and shoulder exercises (one arm at a time), because I prefer to train that way. It allows me to push to the point of burnout, and then, while the tired arm is resting, the other one gets trained to the point of burnout.  If you have access to a Preacher machine, then do use it and mix up your training to see what works best for you.

If the weight is too heavy, you will feel discomfort when fully extending the arm at the bottom.  This full extension is the primary characteristic of the exercise, so lower the weight until you can complete about 10 reps.  If you cheat, then you are headed for the dreaded ‘Golfball muscles’ syndrome.  Biceps are especially prone to this phenomenon.  Calves also.  It will look like you have a golf ball or cricket ball under your skin—the short Range of motion in the exercises done restricts muscle development.  

Finally, I would like to address the angle of the incline required for Preacher Curls.  You can vary this quite a lot (and please experiment with it), but 45° is the absolute minimum, and you can incline right up to around 85°.  Changing the angle places extra work on different parts of the Bicep, so this exercise (when done with an adjustable bench) is highly versatile and a personal favourite of mine.