I use these to improve my strumming technique, especially on acoustic guitar. They are extremely thin and bendy (but, robust), which makes them quite forgiving, and also make quite an audible click on the strings, which I find helps in nailing complex strumming patterns.
They are far too flexible for picking individual notes with any precision, and I don't see how I could ever be use these as a general purpose plectrum. However, as a learning/practicing tool, they work well.