Many recommendations for this question suggest to take Bouncy only if you already have Spreading shots, but I disagree.
First, a comparison of the two:
- Boomerang has a power reduction of 38% (usually on top of the piercing reduction for your forward shot, unless it missed everything) and will bounce once and return to you - in many cases this is the same path, so with a slow moving enemy will result in a hit on the return - the closer the enemy is to the wall, the more likely the return hit. If the enemy is fast moving or not close to the wall, or if you had homing, or if you are moving a lot (e.g. last boss of Junk Planet), the return path can be different, which can reduce the chances of hitting
- Bouncy walls has a power reduction of 50% (again, usually on top of the the piercing reduction for your forward shot unless it missed everything) and will bounce 3 times. The power reduction only applies for the first bounce. Unless you are aligned vertically or horizontally with the enemy, the forward shot is unlikely to hit on the bounces (but if you do, they can hit a slow moving enemy 4 times even if you subsequently move, because it doesn’t follow you)
Bouncy walls are definitely more helpful when you have Spreading Shots (especially the Spread shots can channel through corridors), but given that Spread and Vertical are more common perks, it would be quite rare to not get at least one of them in a run - so taking Bouncy early will work out most of the time; and this is the process I follow:
- Early in the run the only perks I take over it are Homing and Multi (both standard probability perks) and Fast Learner with the expectation of getting it again
- Towards the middle of the run, if I don’t have any Spread/Vertical/Back shots and/or Homing, I might pass on bouncy if it was
- to take Chain provided I already had Shrapnel, Zap or Ice;
- to take Shrapnel / Zap / Ice provided I already had Chain, both because of how they work well together.
- If I still haven’t got any Spreading shots nor Homing when I’m at the tail end of the run, the choice is becomes more nuanced… Is it a boss that I can easily stay aligned with (e.g. Pirate Ship last boss) or with a bit of work (e.g. for Bear boss in Toth, to stay centered above it for a while jumping across?
- In regular worlds the last boss is known so it’s easier to plan (e.g. great for Pirate Ship, mostly useless for Junk Planet)
- For Tower runs / Castles the boss is unknown, so it’s a gamble (and I am more likely to be sympathetic towards decent alternatives like Focus shots)