Just in case there is any confusion out there, the Nightrider devices only affect the air-fuel ratio when the bike is running in closed loop mode (using the data from the O2 sensors to control AFR). The Harley system goes to open loop mode on cold starts and warm-up, and also at higher rpms and higher throttle openings. So while the devices can be very beneficial on a stock bike, modifications beyond a different set of mufflers will probably need something more in the way of tuning. I personally think they are a great idea for those who are keeping their bikes stock, for those who worry about a-hole dealerships and the MoCo screwing with the warranty coverage, or for those who plan to eventually make modifications but haven't decided on particulars yet. They are relatively inexpensive (actually they are dirt cheap for an item sold for use on a Harley), and when the time comes for mods and a tuning device they should be easy to sell to the next guy. I can't think of any real negatives, as long as folks understand what they can and cannot do.
Jerry