We use STM, and chart online. I like it because I’m a scientist, and like to have something quantitative (the temp reading) as opposed to only something qualitative (mucus signs). I find that I rely the most on the temp, and mostly end up using mucus and cervix observations as a double-checking. It has worked really well for us while we were using it, though it was only for a couple of months since we got pregnant after that (we were not following the rules closely that month on purpose, so it was not a surprise).
I also found that the method was easy to learn, we never took a class, just got the self-study book and went from there.
Edited to say: CCL has come out with an online charting software recently, it’s accessible from anywhere (DH can check it during the day

) and it even does interpretations for you. It doesn’t have the most user-friendly interface, but I’ve gotten used to it, and like other parts of it enough that I will be using it again once I need to chart again.