I am a very happy user of Balsamiq. I was able to deliver 20+ rough wireframes in about 6 hours using its interface. It would've taken significantly longer in OmniGraffle and we've been able to use them cross platform b/c of AIR.
At an early stage of requirements gathering they're complementary for wireframes. Revisions/UI specs will be fleshed out in Omnigraffle for greater precision later, but that's after we've iterated with client in Balsamiq, which is what it makes so easy.
A++ would do business with again.