Dunno about most effective place to inject..
But I prefer my quads over anything else. I understand that there's fewer nerves in the upper glutes which would make that a nice injection site but the difficulty of having to twist myself into a pretzel to do it makes it a *ahem* pain in the ass. I like quads better because I have no problems reaching them. I'm also looking into doing my calves for this reason as well.
- Judoman
P.S. As for pain, I just did my second injection of my homebrew fina today and the only pain I get is a 'charlie horse'-type of feeling in the immediate area surrounding the injection site.