After playing around with the slot in the image I've found that, in addition to the cryptic sentences, there are also a lot of useful rewards including:

• trinkets
• Tarot cards
• Playing cards (especially useful the two of diamonds, that doubles your current amount of coins)
• soul hearts

Here you can see my results after playing for a while (I failed to make it blow up by itself, so I used a bomb):

It can become a little cluttered, as you can see. I also had 3 more soul hearts and 1 heart container, but I wanted to kill the boss before finishing my experiments.

After playing around with the slot in the image, and with the help of the wiki, I've found that, in addition to the cryptic sentences, there are also a lot of useful rewards including:

• trinkets
• Tarot cards
• Playing cards (especially useful the two of diamonds, that doubles your current amount of coins)
• soul hearts
• a Crystal Ball (probably on blowup), a usable item that reveals the map and spawns a random card or a soul heart (thanks @Keaanu!)

Here you can see my results after playing for a while (I failed to make it blow up by itself, so I used a bomb):

It can become a little cluttered, as you can see. I also had 3 more soul hearts and 1 heart container, but I wanted to kill the boss before finishing my experiments.