I'm a professional Graphic Designer and I've use predominantly Paint.net for my work for the past 2-3 years. I'm trained in the Adobe suite, but find that my output is much faster and generally of much higher quality with PDN. I don't know of any other professional designers using PDN, but it's an incredibly competent program (with the right pug ins) and would love to see it used professionally more often, because I normally hide the fact that all my work is in PDN.
I don't know what the rules are for posting personal websites, but I don't have my stuff uploaded anywhere else, so I'll go ahead and link it and risk a wrist slap.
Everything on here is done with PDN, except of course the physical art.