I dont really understand the theory behind the barrel, surely its just a cyclinder which the ball rolls through, other than length how can they vary :S
Lots of thing can affect the quality of the barrel which then affects performance.
As an earlier poster says parts of the barrel are unpolished,these unpolished parts can "grip" the balls or cause traction on the way out leading to inconistency to start with.
A good ball-->barrel match can be essential,hence most people have a kit or a few barrels.
Un1tec's as mentioned are hugely popular ahd very cheap,a full set (1 back,3 fronts) could cost in the region of £30.oo.
Never got on with any of the fibre ones myself,didn't see a hell of a lot of difference if any for the high prices...