Very cool game for a number of reasons; and the biggest reason I enjoyed the game might surprise you.

The base game is clever on its own; the way it plays with perspective in new ways makes the game feel fresh all the way through; they don't just simply repeat the same resizing mechanic in the same way in every single level (because that would probably get boring fast). The music and atmosphere are nice, and the game's sense of humor is enjoyable but not overbearing. The experience is pretty short overall; it should take 2—4 hours to beat the game depending on how much time you spend simply exploring the space, and it's a bit on the easier side for the most part.

But after the base game came the part that was surprisingly incredible: the collectibles. Finding all of the collectibles (each type coming with its own set of achievements) was this game's strongest point, in my opinion. Everything is so cleverly hidden, and the different variety of collectibles makes you think about the game's space in ways that don't get old; it really makes you appreciate the design of every level to the fullest (because believe me—you'll be searching every nook and cranny and learn each level like the back of your hand to find a lot of the secrets). Obviously, marketing as a perspective puzzle game is a great selling point, but the collectible-hunting was incredibly well done and quite frankly my favorite part of the game (it also took the vast majority of my playtime). This is where the challenge of the game truly lies because finding all of the collectibles was not easy, but you can definitely find everything without looking up hints and stuff (you can figure out some general patterns for how the collectibles are hidden that help narrow down possible hiding spots).

Lastly, online multiplayer was quite fun. I played several rounds one day and actually had a blast once I figured out what was going on.

As of this review, I do not yet have the DLC, but I made sure to play the base game to the fullest (getting all of the available achievements for that content) before I wrote this.

I love this game. For a comparison of the style, it's similar to The Binding of Isaac--every stage is split up into various rooms with a secret room you can only find by using a bomb in the room, shops, an upgrade room, and a weapon room; additionally, the rooms are randomly generated (unless you decide to make use of seeds after you unlock that option), and you start from floor 1 when you die. But Monolith is far more of a bullet hell experience. Every room is its own miniature challenge, but the bosses and mini-bosses are the classic screen-full-of-bullets experience you would expect.

I like a lot of things about the game, but here are some things I think are particularly worth noting:
--The hit boxes are incredibly tight. There has never been a moment where I've been like "Oh, that hit me!? Come on!" As expected of a bullet hell, you can weave and squeeze between bullets and enemies if you have the reaction time and skill.
--It's charming. I think it's appealing that everything has a personality (even hitting the pause menu displays a picture of your ship doing something goofy). I won't find out more about the characters until I play the DLC, but I'm sure I won't be disappointed.
--Seeding is incredibly useful (any given run is decided by an 8-digit code, but once you unlock the ability to use seeds, you can enter the code yourself to get the exact run every time). For a lot of the achievements, you can just play randomly, but for challenges like completing the run with all secret rooms found or completing a run without getting hit, seeding is your best friend. Without it, some of the achievements would seem too luck-based, so it's a great feature.
--The weapons are great. They each have their own strategic uses, and most of them are fun to use. If you understand the attributes attached to each weapon and utilize them according to your play style, it's a great time.