TL;DR: A an amusing, if very flawed and at times quite buggy 2D shooter with reasonably deep upgrade mechanics and loot system. Not worth the full price. Definitely worth it on sale for some mindless entertainment.
One Gun: Cat has simple but but decent 2D shooter mechanics. Character movement and shooting both feel reasonably good. Sometime dodging attacks (especially from bosses) are based more on luck than skill. There's a fairly deep but not terribly interesting loot system. There's a fairly large overworld map with dozens of areas that have a massive progression of difficulty. Each map area has a series of bosses with difficulty ratings. The starting boss is difficulty 5. I unlocked a boss at difficulty 100,000 before I gave up on the game.
OG:C has a bunch of upgrades, some of which are essential and whole pile of useless ones. There's an item upgrade system which is buggy and can be quite frustrating. All gear aquired by either drops or purchasing can be upgraded. Base grade items can get up to +5 levels and the highest tier ones can be upgraded up to +15. Upgrades require cash (which can be earned in-game and offline, more on that later) and "hashies" which are a currency dropped by bosses. The higher an item is upgraded, the smaller percent chance that an attempt to upgrade it will be successful. An upgrade attempt consumes the cash and hashies even on failure. This makes it an absolute PITA to raise items to max level.
There is a much more consistent upgrade mechanic at the in-game "Gym" where the player can sped cash to upgrade the cat's base HP and damage. This is the most consistent path for progression. The problem here are two-fold: the cost curve gets prohibitively expensive, and it's capped. The max level for Body (HP) and Damage is 3000, yet there are bosses where that amount is barely enough to make a scratch. It took me months of firing up the game every few hours to collect offline earnings (see below) to reach level 3K and even that was not enough when combined with fairly high end gear.
One Gun: Cat has an odd cryptocurrency mechanic where it earns cash offline. It even uses the Bitcoin icon and shows some weird GPU-like machine that generates cash. Unfortunately, this feature is buggy AF. At least 1/3 of the time I fired up OG:C, I get zero cash. The offline earning counter reset, but there was no payout. Worse still, the failed offline earnings got more common after I had maxed upgraded my bitcoin mine. Intentionally or otherwise, the game punishes players for progressing.
There is also a max cash amount. It's quite large ($5,000,000) but also artificial and unnecessary. There's no in-game reason to cap max cash. I'd argue based on the extreme difficulty (in terms of large numbers) of the late stage bosses, there's also no reason to cap the two player stats at 3000. I suspect the game dev had bigger plans but ran out of time and/or interest and tacked on some hard limits.
As I write this, I have unlocked the second to last map section and completely all non-boss areas. I've defeated one boss (level 51,000) but I can't so much as put a dent in the next one (level 67,000). There's an entire section of bosses ranged 60K to 100K that I can't even attempt. There's no real progression from where I am now. I've maxed cash, HP and damage. I have an entire loadout of medium to high end gear, all purple (highest) tier and all maxed to +15. My only option at this point to grind the one boss I can (barely) beat and hope for some exotic loot drops. That is not a particularly fun proposition.
Conclusion: One Gun: Cat is a fine game if you want to play the first few areas casually. Don't bother trying to beat it. Also, the dev released basically the same game under the name One Gun 2: Stickman, which is a bit questionable in my book.
