I had been using boostane continuously for two years, up until earlier this year when I finally got my flex fuel sensor working and switched to E85.
It makes the plugs orange, and it can make the gas gauge act wonky. I think the stuff messes with the impedance in the sender and it won't read all the way full. I always refueled at 1/4 tank indicated. Running a tank of untreated fuel or E85 always restored the gas gauge to normal, so I don't sweat it much.
But those are the only two side effects I have noticed.