This is just my opinion, but I've been a big fan of additives for some time (provided they work) and the only one's that work will completely trash the cats and the O2 sensors on our cars because of the chemicals inherent in it's manufacture- check this brief link out....
Nulon.com
Myself, i use Redline injector cleaner and Lucas oil upper cylinder lubricant. Many other products do work(injector cleaners), but at the cost of not lubricating the injectors. The two aforementioned products are the most complete formulations, and they REALLY DO WORK (from my highly scientific -"seat of the pants" testing and checking mileage from tank to tank over the course of ten or so years).