STS definitely gave me the most arm improvement of all the other w/o's I've done. I had really weak arms as well, but after my second round of STS I no longer had doubts that it was what helped me. The most crucial aspect is the endurance that is built up in the first meso because it helps you to be able to handle the heavier weights in the later meso's. So make sure you are doing as many reps as you can in M1, and going to absolute failure in M2 when she tells you to. Extra Bonus, when you get to M3 the arm w/o's are separate from the other body parts, so you get to really focus on them. But, like I said, for me, it was the endurance build up that helped me to go heavier (thus seeing better results). If your forearms are failing early, it will stop you from hitting the biceps as hard as you'd like.
Another issue, if you feel like you already have enough endurance in the muscles, could be that you need to give yourself more time to build a
really solid muscle base. Oftentimes we are always trying to "tone" and "define" something that isn't really there, so the high rep "cut" or "tone" phase will only come after something has been built, which is done during the lower rep, heavier weight phases.
If nothing else works, I always recommend throwing in another shoulder w/o somewhere during the week, when all else fails, nothing makes the arms pop more than shapely shoulders.
BTW, there was a similar thread that may give you more suggestions on this
http://cathe.com/forum/f97/plateau-am-arm-question-283825/
HTH
