One thing that isn't obvious at first look is that this card is self triggering. Spending a credit from Net Mercur triggers it because Net Mercur is also stealth. You could alternately use that for more stealth credits, or more cards.
The next seriously nice thing is that unlike almost any other "recurring" credit generation this one says "Use credits for ANYTHING". That could mean personal workshop, icebreakers, events, trashing whatever. So the trigger happens all the time.
Couple that with the fact that the trigger happens EACH RUN and you've got a great econ source. Pairs well with Encore Blackstone and Data Breach which start triggering additional runs which net additional cards.
I have heard it said that this doesn't make Mirror better than Astrolabe. I will concur in that mirror relies on R&D runs specifically, but astrolabe requires your opponent to do something specific so you can't trigger it yourself. In this time of asset spam decks and Near-Earth Hub: Broadcast Center domination it might look like astro is the way to go but I think the build-up for stealth is going to enjoy the flexibility of making more stealth runs more quickly and more consistently.