Is it even worthwhile to use an app like that to save a few cents on gas?
Not EVERY TIME you need to fill-up, but it's very good for finding which gas stations in your area are consistently inexpensive, which ones play games with pricing (occasionally cheap to bring-in business, then crank-up the prices). And when traveling it absolutely INVALUABLE for avoiding gas-traps that can be $1 per-gallon more than the gas station half a mile ahead...
If I have to spend even 5 minutes looking up gas prices and driving out of my way to go to a cheaper gas station, it's not worth saving 30 cents a gallon on gas.
At $8/hour (a reasonable minimum wage), 5 minutes of effort is worth 67 cents, making even a 5 cent/gallon price difference worth the effort.
Personally, there's nothing I would love more than an app (or maps/navigation feature) that would show me which cheap gas stations are along my route, rather than a dumb radius search that might tell me to do a U-turn and drive a 10 mile loop to save 1 cent/gallon, or going 5 miles away from the highway, when in both cases continuing on my route for 5 miles to the next cheap gas station is most often the far better option. GasBuddy's map is utterly useless for such things, and would take an hour of clicking-on each pin to figure out the answer to that simple and frequent question.
I see Gas Guru is a solid competitor to Gas Buddy. I'll have to compare their terms and see which is slightly less evil.