I've played through I through III recently, and I'm in the last level for part IV. I was never a big fan of the series before, but playing through them again makes me appreciate it a bit more this time.
As for the music for part II, I gotta agree with Sinistron & Nat. Valis II's soundtrack is decent, but I can't ever recall going "wow" at the music. And the gameplay is by far, far the crappiest of the series. I'd think the only thing that really sold Part II to a large crowd is the whole "anime girls with skimpy outfits" thing.
Talking about Valis, part III gets a lot of love, but IV is quite solid. For a "regular" CD-ROM2 game, it's probably one of the most polished looking titles I've come across. It looks helluva lot better than a majority of the SCD titles out there, with good use of colors & parallex, and even the cut-scenes are tons better than part II. (they must've come up with better compression algorithms by then, cause the images are a lot bigger & varied during cinemas than previous CD-entries, but the loading isn't so bad)