I have never reported reckless behavior by other pilots to the FAA, but I have thought about it several times. The only problem is that usually I will only know the tail number, not the name of the pilot. Since the owner of the aircraft may be different than the one flying it, how does one go about making sure the complaint is filed against the right person?
The FAA provides an online reporting tool here under "FAA Hotline Web Form" (direct link here) I would say it falls under "aviation concern". In your description if you include tail number and time they should be able to figure out who was flying (or in the cockpit if its a 2 man situation). You can also contact your local FSDO and report the tail number and time of incident. Aircraft owners/renters (the people the tail number will trace back to) will more than likely know who was flying a given tail number at a certain time so as long as you know when it happened you should be ok its not necessarily on you to know who was flying but the tail number and incident time is key.