OK you have certainly opened an economy size can of worms here so let me jump into this with my own experience.
As a former truck driver and an owner operater I would always use Howes fuel additive for the simple reason that the increase in mileage would pay for the additive itself anad in the winter time when it got so freakin cold I only came close to gelling my system once because I neglected my fuel treatment. As for what the mechanics have to say my own mechanic said they do have additives in todays fuel however they are low in numbers so keep using the additive that I was using. Also everyone will have their own opinion as well as experience, good or bad, so the best advice that I can give will be do your research, ask for other peoples input and advice, then make your decision based on that.
Good Luck