Leaning can save you more than 20% in fuel for the same speed. There is NO negative impact on the engine especially if you can lean LOP (lean of peak) where the temps start to fall again.
Full-forward on everything normally means about 125F-150F ROP (rich of peak); that's how engines tend to be set up, for max power and extra cooling (plenty of unburnt fuel) during a full-power climb. But it's silly to cruise like that.
The trouble is that to lean correctly you need adequate engine instrumentation. With that, e.g. an EDM700, you can lean until you see peak EGT and then continue LOP; once LOP there is little additional "mpg" gain because LOP engine power output is nearly proportional to fuel flow.
I am suprised someone here leaning a Seneca during climb - Lycoming specifically prohibit leaning above 75% power so the man must have meant a cruise climb at 75% power or less, and with the airspeed being sufficient for cooling.
Carburreted engines are harder to lean because of uneven fuel distribution, so the onset of vibration tends to be seen earlier. The trouble is that if one leans only part-way to peak-EGT, one could end up in the detonation region (~50F ROP) - although detonation isn't actually likely to happen below 75% power.
A good reference for this type of engine management is Deakin's articles at avweb.com.