I recently found a great deal on a used one and put it on my XR440. I lost a noticeable amount of low end, but gained on the top end. Not sure I'd be happy if I didn't have the 440 kit (might be too much low end sacrifice) for my type of mountain riding where low end is needed.
Compared to other headers I've seen, the Powerbomb doesn't appear to enlarge the header pipes closest to the engine. After the resonator and collector, the pipe is significantly larger. They advertise lower noise (a buying factor for me), but it seemed to change exhaust tone more than reduce decibels.
One other consideration is the lack of a heat shield and pant-melting problems, especially if you wear riding pants outside of boots.