I've ridden behind someone that had one. IMHO it was hard to see during the day but better at night. I think the blinking spoiler light is more effective. But if you already have that then go for it. The more light the better.
EDIT: sorry, I just re-read your post. It is for your wife's Shadow. In that case I highly recommend the Kisan taillight modulator. I had one on my Vulcan and it was plug and play. All of the electronics are built into the base of the bulb.