I've found the IRS to be incredibly reasonable. I had a high tax bill due to non-realized gains (sigh...), but was able to get them to do a payment plan at low interest for a couple years. They were nothing but polite and accommodating.

I also spoke with IRS CID on another matter (someone was making death threats to IRS agents, and they had mail through my anon remailer). Way saner than FBI or even USSS. (I had no information, by design, but being able to tell someone that and explain how remailers worked without them threatening to seize servers was refreshing.)