Harmony of Despair has a few interesting ideas, but in the end it boils down to a grind
At first it will be very interesting and challenging. You'll be exploring the mazes and collecting items while battling a wide variety of enemies. There are several characters to play with, each with different abilities and play styles. You will grow stronger in power as you play by collecting and equipping many different items to increase your stats.
You get 30 minutes to clear each stage. When playing coop and you die you become a skeleton. Then you need to meet up with another player that has collected a specific item which can bring you back to life. When in skeleton form, you cannot open treasure chests, and are not suited for battle. If you die again while still a skeleton, the whole party will get 3 minutes deducted from the time available to beat the boss of the stage. So even if you are doing good, you can still quickly lose if other low leveled players are having trouble elsewhere in the level.
Levels are designed for coop in mind, as many pathways and mechanisms require more people to operate and traverse. You can still play solo if you want, but it will be really hard unless you already leveled up a powerful character. Levels become effortless in difficulty if you join up with a group of random players. Many other players are so powerful and fast, that you will hardly be able to keep up with them. Often, they will even reach the end and defeat the boss in under 2 minutes, while you are still fighting and exploring the first couple of rooms. The best way to experience Harmony of Despair is to have a couple of friends to play with, who will be of similar strength.
Not including the downloadable levels and characters, Harmony of Despair only has 7 levels, so if you play online, it is even possible to beat all 7 of them in about 15 minutes. The only thing that remains is farming the bosses for better items, so in the end, you will be playing these same few levels over and over until you finally get bored of repetition. The RPG elements are what bring Harmony of Despair down to its lowest. By becoming too powerful and ganking enemies with other players, you're eliminating all of the challenge and thrills a Castlevania game is known for.