For a free-to-play game, it was great. I enjoyed most of my time playing with friends, and the large quantity of weapons and armors seemed very enjoyable. However, the game has numerous difficulty spikes, along with areas that (thanks to newer updates) almost force you to pay to get the best type of armor you want. However, thanks to the varying damages and damage resistances, as well as status effects, newer players are basically punished for not paying. Although they have made quite a few changes to the game since it began, some of which making it much easier for newer players (such as doing away with elevator costs, and removing "Mist energy", they replace those things with more difficulty spikes.
Also, while the game is nice when you begin, and harder later on (forcing you to play with others, and hopefully good players at that), the other portions of the game (PvP, and the DLC's) are extremely lacking. PvP is a mashup of horrible pings, buttmad children, and people using the game's auto-aim feature. The DLC's offer you two things: A useless couple costumes, along with extra lives and such, or an extremely annoying (and difficulty spike filled) raid on a Gremlin base. However, at the end you get an overpowered weapon, decent bomb, and the ability to make a decent gunner mask.
The story of the game is also lacking. It seems almost like they aren't sure what to do with the game. They have plenty of lore, whether it be about the Spiral Knight Order, or any of their enemies, however they seem to have nowhere to take the game's story from here. The "end" of the game was supposed to be getting to the core to find an energy source and leave. Instead, you get to the core, find out its locked, and have an option (if you save up a lot of in-game money, or pay roughly $7.50) to re-play any of the four boss levels, only more satanic, leading you to believe that you're in that world's version of hell, perhaps. However, the game stops there. It leaves so many questions that I'm sure everyone wants answered. What happened to the recon unit, what happened to the Shadow Lairs, what happens to Warmaster Seerus, what is in the core, or even if you will escape?
Sadly, i've been playing this game for around two years, and in my time they havn't added much in story progression since then. I'll reccomend the game now, but I feel they're losing players, and if they don't finish the story in a good way, or at least expand more, the game will kinda get empty, and eventually die.