There are some fights in the game where Bouncing Flask is VERY good. Single-enemy fights which carry on for quite a few turns. It can also be good with other cards and relics which synergize with poison. That said, any multi-enemy fight is likely to find Bouncing Flask struggling for useful effect. It's important to keep that in mind if you take a Bouncing Flask early, and to make sure to have cards or potions which can address the fights that it's weak in.
