I have not used any of the more expensive lenses, but I have owned and used the 50mm F1.8 II, and then I upgraded to the 50mm F1.8 STM when it came out.
To anyone who owns the 50mm F1.8 II, I wholeheartedly recommend you sell it and get the 50mm F1.8 STM - especially if the unreliable and inconsistent autofocus irritates you (this alone is reason to upgrade).
This new lens fixes basically all the issues with the 50mm F1.8 II (as much as can be expected within its price range).
The new STM lens:
- focuses accurately
- is sharper wide open and sharper in general across the whole frame
- has more aperture blades with their accompanying benefits
- focuses more quietly and smoothly (compared to 50mm F1.8 II)
- can be focused manually more easily than the 50mm F1.8 II, even though it uses a focus by wire system
- can focus on closer objects
- has proper lens coatings with the accompanying benefits
- gains an extra aperture setting of F22
- is visibly sharper even through the viewfinder
That all said... don't expect miracles... it's still basically the same lens, but with a few tweaks.
By the way... I own an ancient 20D, and all these image quality enhancements are clearly visible to me. Anyone with a better camera (I assume 99.99% of you) should see the difference even more.