It provides many advantages, mainly in handling. I have experianced improved steering, improved ride and improved wheeling Characteristics. You can get them at most of the Cruiser specialty shops, or you can make your own (I had this done).
Check out Woody's Tech links for all the info and pictures you will ever need on shackle reversals (And donate some money to birfield.com while your at it)
improved handling is minimal...usually, when doing the reversal, you are replacing spring bushings and such too....and you usually discover any steering looseness and address that as well. Those improvements alone make a big difference in handling/ride.
Most reversal designs are "boulder finders" as well, since the bracketry hangs quite a ways down/ahead of the front frame. Additionally, you must run longer rear shackles, which worsens your ride.