Postal: Brain Damaged is the only game I've played where piss cancelling is a thing - weapons with a slow fire rate can be shot way faster if you piss between shots.

The above illustrates the tone of the game quite well. It's intentionally gross and a little offensive - whether you like it is up to you. To me, personally, it's a little pathetic, and it comes down to two things: there's no intention behind the game's edginess, and there's no cohesion in any of it.
Being provocative isn't bad, but when there's no idea behind it, then it ends up just looking sad. Grossness can be used for contrast when juxtaposed against or replacing traits of purity tied to a group, for example. It can also be an exaggeration that emphasizes something negative, or just a theming tool. Here, none of these apply - the main character whips his dick out, pissing in urinals opens doors, enemies get stunned with urine, there are farting enemies, caricatures of obese people, and you pick up shit to open a door at one point. Nothing is being said with the grossness, it's just there.
As for the lack of cohesion, the game has a very random assortment of enemies - from mutant dogs, through aliens, conspiracy theorists, floating fat dudes, moaning women in bondage and moustaches with cowboy hats, to furries and D&D nerd wizards. That's not a problem in concept, but because every theme only has a couple of enemy types tied to it, the game is forced to mix them a lot, and it just doesn't work, both because of mixing artstyles and abandoning the theme behind each level.

This doesn't ruin the game for me at all, but it's the most distinct thing about it, so it's worth mentioning. Maybe it's obvious if you've played a Postal game before - I had not.

What matters is gameplay, and that's fairly solid. The arsenal you get to play with is decently large and very varied. It comes short because of some weapons being too imbalanced, but the vast majority of them are satisfying to use - they have good feedback and some have animation cancelling like I mentioned at the start. Each weapon also has an alternate fire mode, but most of them are simply not useful.
There are also consumable pickups that try to mix things up. They're nothing crazy, and suffer from the same issues limited quantity items always have, but I guess they do introduce a little bit of variety anyway.
Combat is fast, and so is your movement. You gain speed by bunnyhopping (which is a little janky here, but you do get to zoom around), you can dodge attacks with a slide, and move around arenas with more separated platforms with the shotgun's grappling hook and launch pads present on many floors. The enemies' range, speed and ability to track your movements affects target prioritisation in a fun way.

The enemy designs are pretty good. There are a lot of them, and they are varied - they use melee, close range, slow projectile, explosives, lasers and support abilities, they float, run at you, or hang back. Some also have little complexities added to them - there's an enemy type that can live without its body, there's one that has two stages to it, and there's one that loses control of its weapon after a few shots, giving you a window of safety even when it's firing. Kinda neat.

The enemy roster is well utilised by the level design. Overall, the maps in the game are fairly good, but the opening levels are unfortunately by far the weakest in every way - direction, variety, encounter design and theming - so just know that it does get appreciably better, and continues to improve throughout the game.