Will-o'-the-Wisp's greatest strength is against Anarchs (who currently rule the meta), who have only a handful of in-faction shuffle abilities (Djinn, Paige Piper and Trope), all of which are not commonly seen in the most popular decks. You will very likely see a Levy in Anarch decks but using it just to get a chance of returning a breaker is quite a sacrifice for Anarchs.
This is a lot less effective against both Criminals (Special Order, Hostage, Planned Assault, Express Delivery) and almost useless against Shapers (Artist Colony, Self-modifying Code, Test Run and many more).
Many Anarch decks run 2-3 copies of an AI breaker such as Eater or Faust so the corp might need up to 3 copies of Will-o'-the-Wisp to lock them out.
The timing structure of the run matters when using Wisp as well, especially with the introduction of Political Operative and Councilman with Democracy and Dogma. In step 4.2 the runner decides whether to continue the run after passing all the Ice. Then in step 4.3 paid abilities may be triggered and cards may be rezzed. Wisp only triggered upon a successful run in step 4.4, so it must remain 'survive' anything the runner can throw at it during step 4.3.
It may be worthwhile to note that Wisp is not unique, so you can use multiple copies in a server to either take out multiple breakers or to work around Pol-Op and Councilman.
If you want the effect of Wisp there are also some other cards to consider:
- Aggressive Secretary - an Ambush Asset that must be advanced
- Marcus Batty - need to win a psi game
- Keegan Lane - need to tag the runner
- Corporate Troubleshooter - expensive and needs a program trashing Ice that can be pumped beyond the runner's ability to respond