1. Depends on your system. If you have no amp and listen to mp3s on a budget soundcard, spending $150 on a *cable* is a total waste. If you have high-quality associated equipment, then yes, differences are definitely detectable.
2. Depends on your ears and how you listen. IMO, detecting cable differences, source differences, amp differences is not some mystical thing only some "special" people can do, it's a simple skill that can be learned by anyone. Anyone willing to take the time to do careful comparisons can come to detect differences over time. People starting out who don't know what to listen for in the first place may get discouraged and claim aftermarket cables are bunk-- they're wrong though, but expensive cables just aren't for everyone. People who listen to headphones as background activity while doing other things will have a hard time hearing differences, they aren't on the level that's obvious like that. If you like to listen with eyes closed, focusing on the music, and doing nothing else, you'll get to know your system better as a baseline and be more sensitive to making relatively subtle changes/upgrades like cable swaps.