Daemon Arm

Pros

 * Forever casting beams can easily let you gain the upper hand.
 * They also help you a lot if you are cornered, especially with a lightning beam.
 * Especially useful in co-op, if two wizards get their hands on these, an unlimited mega-beam is at your disposal.

Cons

 * Redundant, because you can prepare your next beam while casting the previous one, giving you effectively unlimited beam duration.
 * It is very unlikely that you will have to cast beams for longer than the normal time without stopping,unless you are using a very weak beam.