I had a friend who had a high end FLAC player and kept trying to convince me it was the best thing ever. Honestly though it didn't sound any different than a really good MP3 to me though.

The actual intent behind MP3 was to sound the same. Just like all later lossy codecs, it uses a psychoacoustic model to remove high frequency harmonics and other "buried" sounds that are supposedly imperceptible to most human ears, in order to save on data. At its max (CBR 320 kbps) almost nobody should be able to tell the difference from full CD quality.

I had a friend who had a high end FLAC player and kept trying to convince me it was the best thing ever. Honestly though it didn't sound any different than a really good MP3 to me though.

FLAC has a different design philosophy. It's lossless which means it literally keeps every byte of data from a CD or similar source but just compresses it. You can still get down to like 1/4 the size (vs like 1/8 for a high end MP3 or 1/12 for an average MP3). Storage was a big deal a couple decades ago but in this age of 4+ TB hard disks there's not much reason not to go all FLAC all the time (except maybe if you really wanna cram as much music as possible into your phone).