Technically, it's two games not one. They just glued them together and removed your ability to JUST play the second one for the TG-16/PCE release.
Here's my (blasphemous) thoughts on Ys. I didn't play these games until 2006 or 2007 because I wasn't interested in RPGs as a kid... even into adulthood. It wasn't really until the last 7-8 years I even gave most RPGs a chance. Anyway, after 17 years of hype I was a little let down. I often wonder if the large number of people who list Ys I & II in their top 10 is attributed to the people that played it back in 1990 and have those epic memories ingrained in their heads, or if I'm just missing something.
At its core, the Ys series really isn't all that special or different from anything else. Ys I and II's biggest claim to fame was voice acting in an era where voice acting on a console video game was unheard of. By the time I got around to these games, voice acting was old news (decades old) so that particular novelty was lost on me. Strip away the voice acting, and what do you have? In the case of Ys I at least, an above average action RPG with great music. And the music really is great; in my opinion, this is by far the series' greatest asset.
The Solomon Shrine section in Ys II, on the other hand, is just inane. That part brought the whole experience to a screeching halt for me. Not only is the design of the Shrine very poor, it's also poorly drawn. It's often times difficult to tell where you are able to go and the whole thing just comes off as sloppy. This might not necessarily be a dealbreaker, but I remember the Shrine taking up a significant portion of the game. How about the rest of Ys II? To be honest, I don't even remember, but I'm sure the music was nice.
If it sounds like I'm trying to rag on these games, I'm really not. I had plenty of fun playing through them, with a few minor exceptions, although it's true I found part I to be the more enjoyable of the first two. If we're looking at all four entries, I'd say I had the best time with III, for all the slack that people give it. Zeta once pointed out that I/II get hyped so much it'd be almost impossible NOT to be disappointed on some level. The opposite could be said of III, and maybe he's right, although I do have a tendency to like when developers take a series and do something a bit different with it (Super Mario Bros. 2, Zelda II, Ys III, etc). I do remember liking the music in III better than any of the others, also.
Either way, Ys I and II is something you probably should own if you have a TG-CD (or PCE). Shit, it's been six years, I should probably go back and play these things again and see if my initial impressions still hold up.