I changed out to NGK Iridiums after I looked down and saw my OEM plugs rusting out one day. Never had a problem and last time I checked 'em they were beautiful.
I run only Champion plugs on everything that comes into my shop. It's not a money thing in so far as Champion makes the plugs for Harley and only single seals them where as the regular Champion RN12YC is double sealed.
Really there is no real reason to change from stock. Personally I run Champions, because I can get them at a deal. Same as oe. I look at it as there are many manufactures for plugs, just like diffrent exhasut manufactures. There is no "right" answer or brand. Just prefrence.