My replies:

1) I wouldn't say that a 580EX II is as powerful as a 400 watt monolight. I use 600 watt monolights all the time (at usually 1/2 power or less), and my 580EX's output doesn't come that close.

2) There are certainly cheap monos, but you don't usually get things like a) color consistency, b) wide assortment of modifiers (although there are speed rings), c) ruggedness, or d) great customer service. While Paul C. Buff's monos before the Einstein suffer a bit from color consistency issues, it isn't terrible. I've owned an Ultra 600, Ultrazap 1600s, an X2400 and an X1600. The very slight color variations (which most of the time go unnoticed) have never mattered to me (and I'm usually using 2-3 monos per shoot).

3) For me, I love Paul C. Buff monolights. In fact, I have to pay more when buying from them because their facility is in TN (so I have to pay sales tax). That has never stopped me. For the money you pay, in my opinion, you simply can't get better products.