This is a frustrating card. Obviously, if you are going to include it at all, it is a 3-of to get it on the board early and to stack up the effect as the game progresses.

However, at least in the games I play as Anarch, the Corp purges viruses maybe once or twice a game. Three times at the outside.

Now, I have been in a few odd games where the corp kept purging to save a key piece of ice from parasite or to floor Darwin, but those are edge cases and not business as usual.

So is it worth 3 card slots to ping the Corp for between $0 to $10 a game, and further to give the Corp control over the tax's trigger (i.e. the Corp owns the decision to purge or not purge and thus whether to pay the tax or not)? Generally, I would say "no".

The next thing to look at is tempo. Getting purged is a tempo killer for Anarch. The Corp takes a full-turn tempo hit, but if it takes the Anarch several turns to recover their board state to equivalent potency the Corp still "wins" that tempo shift. But imposing some click equivalent credit impact helps tilt that over towards a break even. Yet...the 1 click to draw plus 1 credit and 1 click to install hit to get a Fester into play in the first place works against this. So, I would still say "not worth it".

If the card didn't say "IF ABLE", it could be possible to combine this with more general econ denial and functionally turn it into "the corporation cannot purge viruses this turn because they can't afford it". However, it does say "if able" so there is anti-synergy with Vamp, Account Siphon, and Reina Roja type tricks as a broke Corporation might as well take the opportunity to purge viruses thus dodging the Fester tax.

So...yeah...in the current card pool the only compelling cases I see for Fester are 1) you are installing multiple copies of Datasucker and spamming virus counters on them via frequent runs and therefore are more vulnerable to more frequent than usual purges, 2) you are relying on Medium or Nerve Agent for multi access as your primary win condition and getting purged can cost you a game, and 3) you are using Darwin as your primary breaker and getting purged locks you out of servers for several turns while virus counters rebuild.

However, some of the spoiled cards for Anarch reveal that virus counter shenanigans are on the rise, and Fester will definitely become a card to consider in the next few months as the new cards roll out.

until now (I have only played this card twice) it hasn't done any serious financial damage to the corp. But I wonder, if I have 2 or even 3 festers installed, they ability stacks, so if the corp has, say 5 or 6 credits, he looses them all.

I believe they can stack, but you have to resolve them separately, i.e. the Corp purges, then the Runner decides to resolve Fester #1 (the Corp loses 2 credits, if able), then resolve Fester #2 (the Corp loses 2 credits, if able) etc.

3x Fester plus Incubator (in conjunction with a Medium waiting in your hand) or Clot (against fast-advance) might be the best use.