This card is brand new but I'm going to pretend that I only have a mayfly's lifespan to write a review! So here are my initial impressions of Mayfly after jamming a bunch of casual games.
Mayfly is the AI Icebreaker out of System Gateway, replacing the old standard Crypsis, and I think a replacement was much needed. I recently have played some System Core 2019 games and watching the Runner install Crypsis was too painful. It was such a tempo hit to install and use! Mayfly trades off this cumbersome cost with a one-time use effect. I think this design is inspired, showing new players that the strength of the program doesn't matter, as long as it gets you into the server. As an introductory AI icebreaker, Mayfly is a handy tool that will teach new players the fundamentals of the game as they play.
In a competitive sense, I actually think Mayfly has a lot to offer. People have talked about its synergy with Kabonesa Wu and Flame-out, which certainly seems strong. But even in other decks, I think Corps would do well to worry about this little guy. The ability for the Runner to turn a sum of money into an access, no questions asked, is not to be underestimated. I have already had several games where I felt safe rushing out behind two pieces of ice, only to have a Mayfly and a handful of credits ruin my plans. Mayfly will surely have a place in decks looking to land critical runs, such as those using Apocalypse, but I think it may be more generally useful in the early game to keep Corps honest. I kind of live in fear of this bug.
So I guess my message is this: Remember the Mayfly! An install and an Overclock could be all it takes to break a critical early-game advantage, like an Atlas counter or an ARES score. I think Runners should consider slotting one of these, as it can be helpful at all stages of the game, if you have the credits.