The most important thing to know about the EF 50mm f/1.8 STM is that it's a real, honest-to-goodness fast normal prime lens—most of its strengths and limitations are intrinsic. I've used mine professionally as a photojournalist since whenever they were released. It hasn't broken. I've made enough using it to buy dozens of them—but I haven't had to!
The major functional difference between the STM and the other 50s is ergonomic: it's smaller and lighter, but it's also substantially larger and heavier than a body cap.
One-shot autofocus is extremely reliable on my 5D Mark IIIs—I don't think I have much if any AFMA dialed in—but it does not focus quickly from distant subjects to very near ones. AI Servo is also reliable, but the stepping motor has trouble keeping up once the subject distance closes in on the area where most of the focus throw is, maybe 2 meters and closer. By trouble I mean <100% keeper rate.
You can use it wide open, but not a good idea for off-center subjects.
You can use it at f/2 and get a nice mix of sharpness and vignetting (if you like vignetting.)
You can use it at f/2.8 and have pretty much you'd ever need, depth-of-field notwithstanding. If you want to use it as a walk-around, It's sharp enough that you can crop the difference from 50mm to the long end of an f/4 zoom and have sharper image with comparable or shallower depth-of-field.
You can stop down more and, well, get the normal effects of stopping down more.
I love how the images look much of the time. If you need one, get one. If you want one, want it for the right reasons—it's a great tool. If you have your heart set on something more exotic, don't bother.