i had the dg r-series slip-on on my xx for the money i paid for it new ($160) it was very noticable power gains over stock, but one of the loudest exhuast i've herd, to me it still sounded good though.
I had one major problem the pipe section that slips over the stock head pipe was warpped when being welded to the rest of the pipe, so instead of being round it was ovaled. i had to hunt down a small pipe expander and try like hell to round it out again with out going too big. once i got that fit and finish was good, one the first ride some of the packing was being blown out around the end cap though.
i would still recomend the exhaust