[nualle]

Granted that my practice with a bullwhip has been minimal, this is what I think: the whip would be mostly useful as pre-fight intimidation. If the BGs facing you have long enough attention spans (and are far enough away) to be impressed by a few good cracks of the whip, you might win the fight by averting it.

Once a fight is begun in earnest, I can't see the whip being very useful. Chances are if you make contact with anyone with it...
A) You might raise a flesh wound, which enrages the receiver (who is necessarily within a fairly small radius of you already). Even if you whing the guy's eye, in the heat of the fight, that may be more a distraction to him than a stopping wound.
or
B) You might wrap the whip around the BG's limb(s). In the latter case, even if you've taken the BG down (got both legs), you've lost further use of your weapon (unless you invest time you probably haven't got in getting your whip back). Now, unless you've got backup, you're SOL if there are more BGs and/or the first one refuses to stay down.
or
C) In the 1:1,000,000 case, you might wrap the whip around a BG's neck. Since cast and pull (my terms -- I have no formal training on the whip) are natural motions in using the whip, you might kill the BG on the pull. You're still SOL if there are more BGs. Furthermore, if you're that good with the weapon, you'll have a hard time defending yourself in the second half of the fight -- the part in the courtroom.

Personally, I'd rather have a 9mm. Lacking that, a nice bo staff or walking stick.