Summary: Mediocre. If you like this genre then go for it. If you don't, there's nothing for you here.
I completed Ys I in about 4 hours on Easy difficulty. Why easy? Every JRPG I've ever played defines difficulty as how much time you have to waste, not how complex the game is. Ys is no exception.
Gameplay: If you don't want to play with a gamepad, do not buy Ys. You will hate it. The combat involves running into enemies and attacking automatically. The success of the attack is defined by the angle at which you attack. It's an interesting mechanic that would otherwise get boring if not for the relatively short story. Aside from bosses, every single enemy is exactly the same but with a different sprite and stats.
The bosses vary, but their behaviour does not change based on difficulty, the only things that change are stats. All of them but the final boss had some reliable skill-based mechanic to beat them. The final boss was frankly garbage. You cannot reliably dodge and random floor tiles disappear without warning causing instant death. As such from what I've heard, the final boss on harder difficulties is nothing but luck and repetition.
Your level caps out very quickly and there are no more than 5 total item tiers (and only 3 item types: sword, shield, armor). Amusingly, the typical JRPG feeling of grinding is still there despite this. It's probably because, as above, every enemy is identical aside from a different sprite. There are no skills, no spells, items do nothing but increase your "run into people" power.
I almost quit when I came across a cave in a low level field. I entered and in good JRPG fashion was killed in one hit and bounced back 30 minutes to my last save (autosave is not a thing). Flawless design. Was it some horrifying boss monster I had roused? No, it was a carbon copy of the monsters I was one-shotting in the field but with a different sprite. Was I given warning that this would happen? No, there are no levels or stats displayed about monsters until they punch you in the face.
You will have multiple moments where you need to either look up an FAQ to have any idea how to proceed (which will probably involve a lot of backtracking because you failed to talk to one person you had no reason to talk to). There's a difference between holding a players hand too much and straight up not telling them what the game wants.
Story: Typical JRPG nonsense. Deity disappears. Ambiguous anomaly appears! Monsters appear! Stereotypical motives. Stop bad guy with a name that sounds stupid to someone that actually speaks English. Sounds like every other JRPG I've played, though I'm sure some weeaboo will claim the story is a masterpiece. There's really not much else to say.
On the plus side, the actual porting and extra features such as switching between retro graphics/musics and remastered, etc. are all very good, but not a reason to justify what is in essence a bad game.