I only have Valis I and III, and am only close to beating III (Valis II is on the way). I have this sneaking suspicion that one of the series' hallmarks is spawning enemies right at ledges in order to knock you back into a cheap death. At least, this seems to be one of the defining features of Valis III!
I suspect I might like Valis I more, but it's too early to tell.
I watched some longplays of Valis II on the home computers - the story is delightfully dark and bleak (and the booming FM synths of the X68000 help with this atmosphere), though not so much that it is some random gore/sex fest. It's a shame that the game looks like it controls about as well as a three-legged horse in a Chuck-E-Cheese ballpit.
Valis II has short, rather basic level design. I feel that, with minimal extra effort, additional stages (or, extended stages) with more creative platforming challenges would have elevated the game. I am not even asking for new graphics/themes...just simply more interesting/challenging moments.
I am in the minority, I think, but the PCE Red Book is fantastic in Valis II and really helps foster the perfect atmosphere for the game. I just wish some of the tracks were longer. BUT EVERY SONG KICKS MY ASS. Sadly, most people feel otherwise (they feel every tune sucks my arse).
Anyway, I'm not a fool, I know that Valis II is not a AAA title.
However, it falls into the category of "games you love"