I actually have a pretty nice injector, but have not used it a single time. I have been meaning to, but just haven't tried it yet.
I typically brine 95% of my butts and briskets and as a result, don't really need to inject. The one time that I didn't brine a butt because I did not have enough time, I was disappointed. So, the next time I am short on time, I will definitely inject instead of just throwing it in without brining or injecting.
I haven't done any experiments, but from what I have seen, brining and injecting achieve close to the same results and doing both can only make things better, I would assume. Too many experiments and too little time.