Right now I'm playing with them and find out that I don't use them everywhere and aren't so OP as many would say. They're also not redundant with stack inserters because both have pros and cons.
To operate between machines and belts, inserters are more useful then loaders but loaders are very handy with routing items on belts. If we get fully supported loaders with ability to connect to circuit network to set filters via signals, I think that it will be great addition to gameplay.
Here two examples when I'm using loaders (still experimenting):
Belt buffer:

Router:
