Pros:
-Very good damage/AP
-A melee weapon that doesn't rely on Strength at all, good for a high Agility/low Strength character
-Concealable: monofilament whip + fingertip compartment is a classic
Cons:
-Requires the Exotic Weapon skill, which can be a major drain on skill points at creation.
-Illegal, so not appropriate for all situations.
-So dang deadly that you might attract undue attention, in the same way that busting out a grenade launcher in a knife fight would.
-Glitches will MESS. YOU. UP. (I'm considering making a monofilament whip character with only 1 Edge named "Tempts Fate")
-Other runners will snicker behind your back for embracing the cliche.
It's an iconic weapon, it's supposed to be good, and if you build a character around it the monowhip is definitely scary. But it's not as deadly as an assault rifle, not as pragmatic as a pistol, and a jacked-up troll adept or ork street samurai or what-have-you can probably do more damage with a combat knife. It's a good weapon, but by no means the end-all.