Through Google you can find stories from a scout sniper who carried it in Vietnam. He never fired it in combat, but said it was pretty reliable and fun for the gee whiz factor. He liked the fact that it's stealthy, the "bang" happens several meters away from the gun, and it won't explode if you fire it with a clogged barrel, an issue when you lie in mud all day.
I remember a Man from U.N.C.L.E. book where Napoleon Solo managed to kill some THRUSH agents because he fast talked them into thinking his gyrojet pistol was a cap gun.
-1 because this clip is too short.. it's pretty cool to watch it fire.
A couple years too late, but I came across: http://www.militaryphotos.net/forums/showthread.php?38550-Gyrojet- pistol-issued-to-SOG-in-Vietnam
The first response has a few first hand anecdotes.
A cool thing about the gyrojet is that its fatal flaws were a result of 1960's era ammo manufacturing, so we're just a DARPA contract or a few rich investors away from badass rocket rifles. Its sci-fi awesomeness is undiminished... imagine something the size and weight of a .45 that fires mini RPGs. IT COULD HAPPEN PLEASE PLEASE PLEASE