I agree with Midwest.... I run a HRS45 on my fatboy with .510 cams and S&S gears, and the throttle response is unbelievable, likesomeone elsesaid, she just can't wait to get up and go!!! I have never had the least little problem with it either. $200 bucks for a 42 if there's nothing wrong with it, seems like a good deal to me. Seems I gave about $450 for my 45 out of the box. Not sure but I think they make a 48 now. Good Luck with what ever you decide, but I personally think you'll be happy once you do it and get some other mods done.

 

I guess I will reiderate everything that people have said here. When I put my 42 on I didnt notice any difference...until I grabbed a hand full of throttle and IMMEDIATELY took off!! The throttle response on this is night and day compared to stock.
Another feature I like on these is the idle adjustment. Just a twist up or down depending on the weather has been really helpful.