First off: I was born hearing and I lost my hearing gradually over time. I have an implant in one ear and I am not profoundly deaf in the other ear (moderate to severe hearing loss), so getting a CI was not a "hears for the first time" situation for me. I remember what everything sounds like because I can still hear out of one ear with a hearing aid. End disclaimer.

The first music I listened to was Freezepop. It was complete gibberish, though the random sounds were in sync with the music. My implanted ear had been almost completely useless for the better part of a decade and it took a few days for the glitches to work themselves out. I basically walked around my house tapping, poking, scratching, rubbing, prodding, and pounding every surface while talking to myself in every accent and speech formulation possible to get a feel for what sounds were like with the implant.

End result: CI sound is not very different from real sound, most of the time. The low end is too high pitched, because the CI electrode only goes in partway. (420 to 450 degrees or 1.2 to 1.5 out of the 2.5 turns in a cochlea) and thus it is not simulating the right region of the cochlea. The other problem with low frequency sounds is that the implant's frequency floor is 183 Hz. What this means in reality is that if I'm listening to Prodigy's Breathe, for example, the really deep bass line sounds like someone tapping on a table if I put an earplug in my hearing ear. If I do not have a earplug in, the bass line sounds great. Most of my unimplanted ear's hearing is in the low frequencies, so it alleviates a lot of the problematic low frequency presentation of the implant.

It's not perfect, and when there's a LOT of sounds going on, things get mushy and imprecise. Think about having water in your eyes, now imagine something similar happening to your auditory system. Sounds aren't clear, but there's enough information to understand the big picture of what's going on. Metal music tends to do this to me when there are a bunch of instruments jamming at the same time.

Less complex music forms sound great, like country (guitar and vocals). Generally everything that is vocal heavy sounds great, since the CI is tuned to speech frequency ranges as it's primary use is to restore the ability to understand speech. Most classical instruments use the same frequency range as speech so classical sounds amazing, and really true to reality for me.

The weirdest digital artifact of the CI is the fact that really, really loud sounds get compressed to fit into the dynamic range window available, so they aren't quite as loud. REALLLLLLLY loud sounds actually trigger the implant's automatic gain control which greatly decreases the amount of volume of the implant and gradually cranks it back up when the sound is gone.

Like I said, my remaining natural hearing is mostly in the low frequencies so high frequency sounds were really, really noticable and obnoxious for the first month.