Haven't 100%'d it yet but it's pretty good. The Splatoon team did a really good job of creating a rougelike out of splatoon and I personally found it extremely enjoyable. It feels really good getting more and more powerful with all the weapons having satisfying potential to get extremely strong. I also found it very well balanced, none of the weapons stood out as particularly weaker than others (though some are definitely on the strong side looking at you blaster) and the chip balance I thought was especially excellent. I've had runs where I used mostly purple chips, blue chips or yellow chips while skipping out on direct damage boosts and all of those runs not only felt winnable but were very fun too. Especially the yellow chip runs I didn't expect the luck based ones to be broken but they are! I also found the enemies fun to fight, they're designed in a way where all builds can be viable against them which rules. Like ink mines are great, I didn't think they would be great in a pve mode but they are! Sprinkler sucks tho </3. I also found all the interactions between the characters really cute they were lovely.

Now for the criticisms, my main one is the lack of variety in the mission objectives. There are a total of five mission objectives, destroy portals which is the most typical one which I always find myself enjoying, turbine tower which is alright but can be a bit annoying on weird map layouts, 8 ball which is fine I guess it's okay, chase the fleeing enemies which is also fine I guess, and splat zones which ranges from really fun to absolute hell on the worse map layouts for it. These modes are all good but they're all that's there, there is map variety but even after a few runs I started to see constant map repeats. I feel like the DLC would benefit greatly from 2-3 more objectives, such as maybe a simple get to the end goal or maybe another mode based on a multiplayer mode like tower control. Maybe, I'm not a game designer these are just random ideas I came up with while writing this. I feel like just a few more objectives would contribute greatly to replayability. Also minibosses could be cool just saying plus a bit more enemy variety.

Also, only 3 bosses isn't great. That contributes to runs feeling samey too since they don't change much between each encounter either. There just isn't much variety in general which sucks for this kind of game.
ANOTHER thing uhhh I thought the story was kinda lame especially compared to octo expansion. My thoughts haven't solidified on it much but it just didn't stand out very much and felt a bit rushed to me if that makes sense. I haven't finished the postgame yet though but I can't see that changing my opinion much.

Overall Side Order is really good but it could be so much better if it had the budget and content of a full game instead of a DLC. There's probably more thoughts I have but I can't remember them right now. If I had to compare it to any other rougelike I've played feel like it's most similar to Hades in how it's structured and plays out.