Got interested in this a while back, so, I've been pinning Tris and upper pecs pretty consistently. Haven't done a glute shot in months, in fact.
It works somewhat, you'll see growth at first, because of the swelling that will accompany pinning a virgin muscle. Fascial stretching or anabolism, or both, who knows? After that, growth rate stays the same...unless you increase the injection volume, IME. You'll still grow, of course, just at the regular rate, seems to me. But it's good for a quick bump in size when you're stuck.
Beware...It can hurt post-inject and you'll likely catch a bleeder here and there and it leaves a bruise.
One thing I've noticed is that you CAN pin a flexed muscle. Doesn't hurt. If you can't reach outer tris for example, flex the bicep of the arm you want to pin, reach over to where you want to pin and hit the exact spot. Then apply pressure to the spot in a relaxed state to limit bruising.
That's my personal experience with it, so far.
Other info I've read...Dr. G. (of the Dr. G. Q and A thread), says he prefers short esters, particularly NPP, and has a study that indicates site growth...with chickens though, but it's something.
Another member here (maybe banned now?) prefers even shorter esters, like Suspension and Tren-no-ester, timed with an insulin spike. Banned or not, Dude has good arms and shoulders.
Then there's JJB1, whose arms testify that something is definitely working.