I will admit my Garrett is quite good at descriminating against garbage. I can set it for a nickel and it will pretty much stick to that. Serious MD hobbyists don't want to waste their time with fruitless digging so they get a good detector and master the technique of listening to the tones and reading the indicator.
I recommend you go to a beach and bury some various coins, pull tab, a spoon, even iron. Keep them buried at the same depth and separated a couple feet apart. Practice using the detector to learn how it reacts to the various metals and play with the controls to learn how to tweek it. Then bury the same items deeper and repeat the process.