The best mirror matches are among my favourite matchups to play in all of Magic.
At their best, mirror matches take all the best and most challenging aspects of playing a deck and amplify them by turning them in on themselves. Card assessment becomes harder when you know the opponent has the same tools as you, and the question of who is the beatdown is much less predetermined than when you get paired against a fundamentally more aggressive or controlling deck. The Temur Energy mirrors were always deeply fascinating to me due to the manner in which every threat in the deck was capable of varying in power level so wildly from one game to the next, for example, and really highlighted the strengths and weaknesses of each of the threats in a more general sense. A huge amount of what you could want to learn about Temur Energy, about how the cards themselves will line up against different decks and the things you have to be used to weaving past, you could learn just by playing mirrors.
Whilst the Mono-Blue Aggro mirror definitely has key moments and play patterns that are unique to the matchup, if you're looking to learn about risk assessment with the deck, about tempo and how to both maintain it but also how to play from behind, about managing your resources based on what your opponent could have access to, then the mirror is where to learn these things. Further, a lot of Mono-Blue's strength comes from tripping its opponents up, passing with open mana and challenging the opponent to decide on a play, and playing the mirror is an effective way to understand what it is like to be on the other side so that when you pass the turn to your Sultai Midrange opponent you'll have that much of a better idea of what is going through their head as they stare across at your two untapped Islands. So let's dive down into exactly what makes this mirror match tick.
Much like how in normal matchups there are games where you have Curious Obsession and ones where you don't, that is also the case in the mirror match, only in this case it's a threat that could hit the battlefield on either side. Weaving these early turns makes for a lot of tension, whether you're presenting the Obsession or attempting to resist one, and lays the foundation for the rest of the game ahead.
So you have a Curious Obsession in your opening hand. If you're on the play and have a one-drop, then this is all relatively straightforward: disrespect Spell Pierce, slap your enchantment on your creature, and go to town. Each level of abstraction from this situation makes things slightly more challenging. When you don't have the one-drop, then aiming to suit up a Merfolk Trickster is a perfectly fine plan too, but is a bit more vulnerable if you're not careful because your opponent can easily have two mana up on your Obsession turn. The situation is much the same if you're on the draw with a one-drop: you can easily find yourself looking across at two open mana whilst you stare at the Obsession in your hand.