For me overclocking is usefule in certain situations suchs as when mining I like to increase the core clock of my GPU's, when benchmarking I like to overclock the Gpu and CPU, when gaming I usually do a small GPu overclock, and lastly when a system gets older and requires a boost of speed is a good time to overclock your system.
Life span really depends on how you overclock your device voltages and the temps. If your temps and voltages are much higher then stock that can reduce your devices life expectancy. In my case when I overclocked my i7 920 from 2.67 to 3.65Ghz it only required a .01V increase and temps were only 5 degrees higher then stock so i dont thing life expectancy will be affected