Well, having spent the better part of a few days with Valdis Story, and I'm sure I'll play through a bit more, I feel like I'm finally ready to give my opinion on it.
Should you buy it? The short version is "no." The long version is, "Yes, but..."
Let's quickly figure out why you're here first - you heard it was either difficult, or Metroidvania, right? Well, the game isn't hard, and what's Metroidvania about it is a Frankensteined mess of some of the worst aspects of a Metroidvania game. That's not to say it has no good qualities, but the bad outweigh the good by far.
Bad platforming seems to be a staple of Metroidvania games, and Valdis Story most emphatically doesn't disappoint. It's the worst, and the controls just aren't up to the task without extreme frustration. Some of the most egregious of these processes not only involve extended sections of bad platforming, but bad platforming on small, moving ledges, again with conveniently placed enemies designed to knock you back off, falling down a floor or two, sending you back through rooms you've already cleared to re-clear them and try again. Will you grab the ledge, bounce off the wall, or fall harmlessly down it? Hard to say, but it'll probably annoy the hell out of you. Really, many of the enemies are obviously placed with the sole purpose of knocking you off the platform, as they pose no real threat, but are placed in such a way that actually getting to them without being knocked off is the real challenge.
Most of the enemies are fairly well-rounded, and bring some interesting mechanics. Again though, they're not really challenging, and most can be pretty easily dispatched by charging in head first and mashing buttons. Of the few that don't fall to such a basic attack, some of them are rather entertaining to play, if not a bit cheap in some of their moves. There's something about an enemy going invulnerable and casting a nearly unavoidable ice storm that freezes you, forcing you to waggle an analog stick to break free, only to be hit with yet another ice attack, become frozen directly after escaping the first, and going through the waggle motions again.
The bosses are the real highlight of the difficulty, but those... it's really tough to say they're difficult. Some run the gamut of being mildly challenging to somewhat difficult, but it's safe to say all of them generally fall to the same tactic of avoid their 2-3 moves, grab a few quick hits (or stunlock them, if the fight permits it,) run away, and repeat. They become quite predictable quite quickly. Thankfully, the game incorporates a Devil May Cry ranking system based on a few factors, speed being one of them, that encourages risky tactics to fell them quicker, which grant better rewards based on your rating at completion. An especially nice touch that I really appreciated, and some of the battles grant significant rewards, such as massive amounts of experience to stat bonuses.
While we're on the topic, the combat system for melee is quite basic, but typically does its job well. There are quite a few aspects of it, however, that get more in-depth, and the game is very tight-lipped as to explaining what they do, and why you should care. Having a browser open isn't a terrible idea. Unfortunately I can't give praise to the magic system. Many of the spells are rather useless, and though you will NEED to use some of them to accesss secret/hidden/progression areas, there's a good chance that's ALL you'll use them for. The control scheme for them is atrocious, where holding right trigger plus a directional area on your D-pad or left analog stick is disgusting, plus the fact that holding down on your D-pad or analog stick causes another reaction, and shifting spells is typically done with right trigger+B, at which point you have to cycle through them all until you find the desired spell... it's just bad. Really, really bad, and the spells themselves are pretty underwhelming. All said and done, melee works rather well, but avoid magic like the plague.
Further, the map is almost completely useless, which is a real sin considering the game involves large amounts of backtracking to find places you couldn't previously access. The UI is bad and could use a severe overhaul, and for whatever reason, when you've triggered enough hits to access your "combo breaker" or whatever it's called (also VERY underwhelming) it lets off so many sprites you actually think there's something going on that requires your attention in the upper-left of your screen.
I won't touch on the story too much, since there's not much more you can say outside of convoluted, difficult to follow, but still dull as hell. Angels and demons are waging a war, you've been caught in the middle of it while attacking a demon ship, neither the angels nor demons like you, and you need to save the day while reuniting with your crew and befriending other citizens caught in the middle. It's not interesting, and if you skip through the dialog, I wouldn't blame you.
All that being said, the game has enormous amounts of replay, unlockables, secrets, etc., with multiple ways to play. If you're a person that doesn't mind frustration, and enjoys the Metroidvania style of play, despite its numerous flaws, it's a solid game that will probably give you quite a bit of entertainment. If any of that doesn't appeal to you, I don't think you'll appreciate Valdis Story. The game has some great ideas, and moments of good fun and combat, but those moments are few and far between, and require enormous patience on the part of the player.