A while ago, I asked about the E3 spark plugs, and just tried them in my string trimmer and push mower.
In the trimmer(poulan pro 125)(replacing a champion RCJ6Y plug), it started in 2 pulls vs. 10, I didn't notice any power gains, but it does idle a little higher and it doesn't bog down under throttle for the first minute anymore.
In the push mower(cub cadet 6.5 hp)(replacing a torch plug), it starts quicker(electric start) and has a noticeable power gain.
I won't be checking for any reduction in fuel usage, I have no base to go from, and I burn less than a gallon or 2 in each per year. The starting, idleing and power gain could be from being new plugs, but I'll keep an eye on them and report back if I find anything else.