I had a quick browse and their stuff looks good and gets decent reviews. However, for pretty much the same spec and arguably better quality then a decent independent such as TorqueFlow at MSL will do as good a job at a lower cost. You get the personal touch and a bespoke exhaust.
Agency Power are a good American brand. Quality is good and the exhausts sound nicely tuned; I used them on my BMW's BUT, they don't tend to cater for LHD cars, therefore I'd not advise using their headers or downpipes...basically anything that having the steering over the other side will interfere with. Secondary cat-back on C63 should be ok.