Okay, so, figured I'd do a bit of an update here.
Testing with this game continues, as it also continues to update, and, well....
After a big pile of hours with it, my current thoughts are as such: This is quickly becoming the best game of this sort that I've played since Binding of Isaac and Spelunky.
Now, it does still have a few wonky balance issues.... bullet-based weapons are pretty bad right now, and the character "Rebel" is all sorts of terrible.... but other than that, this aspect continues to improve. Some of the oddness with the controls seems to have vanished, though the ability to CHANGE them up is still not there. But the aiming and such is less.... all over the place, which had been my main issue.
A few of my previously-made complaints:
Level Exit Portal: Okay, this hasnt been changed, and likely isnt going to.... I still generally dislike the way this works. That being said I've at least found a way to keep it from thwarting me and preventing me from missing out on items, which is to leave one enemy, probably a bandit (this game's version of Goombas, basically) alive back near where I started the level, and kill everything ELSE, and then make sure to get items, and THEN go pop that guy and exit. It's pretty easy to do this (usually) so that works out well enough.
Level-ups: These have gotten better. Much better. There's still a couple that are just outright useless (like the "stress" one) but all in all, this mechanic is now coming along nicely. It aint perfect, but balance tweaks continue to be made.
Screen-shake: This.... I dont even know, just stopped being a problem. I left it on 50%, and now it just BUGS me if I turn it off. 100% is still a bit too much though, and certain very specific things make it go berserk (good luck seeing what's going on when firing the minigun! Or when the "Big Dog" boss explodes and blows up the universe).
The things that were good about it continue to be good and get better. The character variety really adds alot to this, and each character really is very distinctly different. For example there's one, Robot, who can actually eat weapons. When you do this, you either get a big blob of ammo, or more rarely, you might get some health refilled. He can really get quite the advantage by using this well and making good choices with it. Or "Melting", a character that is.... well, clearly melting. He's a really odd one; he gets a whole 2 HP max.... anything other than a maggot (least damaging enemy in the game) will kill him instantly unless you get the Rhino Skin mutation, but his ability is.... well, crazy. What he can do is simple: Hit the button for his active ability, and any corpse or broken object on the screen explodes. Violently. These blasts are POWERFUL. You can seriously kill the funky hell outta things with these, and you can explode the entire screen at once if you're doing it right. It gets even loopier if you get the stupidly-named "Throne Butt" mutation, which is the mutation that has an enhancing effect on your character's active ability, different for each character but generally always good. With him it just makes the explosions even bigger, and REALLY damaging. So he ends up being the perfect example of a very high risk yet very high reward character.
And what's interesting about the special abilities is that there are never, ever any limits or cooldowns as to how you can use them. If you use Melting's uber-blast, there isnt like some silly 20 seconds that you then have to wait to use it again. You can use it again as soon as you are ready to. This applies to every character, and goes a long way towards enhancing the feel of "freedom" that you get when playing this. It really makes me think of alot of older games; games like Super Mario Bros and alot of those older ones, when your guy had a special power, chances are you could use it as often as you wanted in many games, no silly cooldowns like there are in today's games, which can often feel very arbitrary. The lack of cooldowns in this really just help alot, and the different abilities vary wildly from one character to the next.
And what i'd found before that I'd thought was a hidden level was not actually a hidden level, but a "crown vault". Crowns are interesting. When you grab one, it brings up a screen similar to the "choose a mutation" screen upon levelling up, except that in this case, you select from a list of things that for the most part all seem to be.... well, bad for your health, I'll put it that way. Stuff that makes things harder. Right now, the crown mechanic is not entirely in the game yet, so the crowns will do these things yet have no overall purpose, but it seems there's alot coming to this bit later on.
ACTUAL secret levels though, are indeed in there. I wont spoil these, but I will say, good luck figuring out how to get to them. Just like in Spelunky, the methods of reaching hidden levels in this can be pretty funky, and you have to know just what you're doing in the same way that you had to know in Spelunky. The hidden levels are worth it though. One of the two is pretty easy, and can be a great way to heal up a bit, get some additional ammo, and maybe another weapon choice... while the other one is, well... the other one, I'll just call "MURDER DIE HAHAHAHA", because that's what happens to you when you enter it. I've managed to beat that level, but it's very.... very hard. The specific reward for that one is only half-implemented right now, but I'll say that the level is very fun. And the method of GETTING there is VERY quirky. You'd have to check the Wiki for the game to find that one out. I dont think it can currently be figured out otherwise, there's no hints to the screwy method in the game right now.
The ONE problem I have with the game at this point is that the worldgen can be.... wonky. There's a recurring problem where, every now and then, you'll begin a level in a REALLY bad place, the sorts of spawns that tend to just make you immediately dead, with little you could have done to stop it. This can be pretty frustrating, but unfortunately is just part of dealing with a beta. The devs are well aware of this issue though, so that's good.
Overall though, yeah.... this one really is turning out to be meeting it's potential, and it's still got a long way to go until release. There's a big list of features the devs want to add, and it looks like this should be something really special by the time it releases.
At this point, yeah, I can totally recommend it.
....also, that sledgehammer is still a superb weapon. I just dont get tired of that.