Butterfly Swords is interesting as guardian weapons go. In terms of investment, it's a 3-cost card that takes up 2 hands. The resource cost is average as weapons go, but its cost means you won't be holding a second item without a Bandolier.
It's a 2 XP weapon, meaning characters with secondary Guardian access ("Skids" O'Toole, Diana Stanley, Joe Diamond, and William Yorick, along with Lily Chen on release) can use it, along with all of the full guardians. It's rather cheap in terms of Resource cost, only requiring 3 resources to be a permanent fight option.
The fight option itself is also interesting. The first part is simply a fight at +1 attack - not a particularly strong option on its own. Any fighter wants to be dealing extra damage to actually handle larger threats, and non-fighters will need a bigger bonus if they want to reliably land a blow. It's a small bonus, but not the focus.
The second part is the part that you're paying for. After a fight with the swords, you can exhaust the swords to fight again, adding your to the test. This means that you'll get a second check (regardless of if the first hit or miss), which can be used to target the same or a different enemy, and which will be at a reasonable bonus. (The only Guardian with less than 2 is Leo Anderson, and everybody else will get +2, at the least.)
How good is that in practice? If you're only dealing with a single big monster, it's solid. You can take down 3 HP targets with a single attack, or get some decent attacks on 2 targets should their health be low. If the enemy has 4 HP, then you can match any other +1 damage weapon by taking two attack actions at it. 2 extra damage every turn isn't something to scoff at - at the least, you can compare it to firing off a +1 damage weapon twice every round, on a tool that doesn't require ammo.
Conversely, this means that the weapon is weaker if you're ever spending the entire time attacking, or don't have enough to make an extra attack consistently land. An extra attempt each sequence does mean you might be putting yourself at further risk if Retaliate is a factor.
More importantly, how does it compare to the other guardian weapons at that cost?
- .32 Colt (2) - A bit higher cost, and an extra hand slot is taken, but unless you're spending all 3 actions attacking, you're coming out ahead. Then again, unless you're Mark Harrigan, you're probably not looking at this to begin with - and even if you are, it's probably not one of your upgrade options.
- .45 Automatic (2) - You're comparing against the 'core' firearm. At 1 cost less expensive and winning over two rounds of fighting, if you can spare the hand slot, this probably comes out ahead.
- Blackjack (2) - Blackjack has always been a reasonable backup weapon at 3-4 player count, so in those circumstances, it's worth consideration. However, at lower player counts or as a compromise option, Butterfly Swords isn't bad - having your attack broken into two fights means that you risk more collateral, but will also divide the risk into separate instances as well.
- Survival Knife (2) - The Survival Knife tends to be a more specific weapon, useful when you plan on countering enemies that attack you. It's not a bad weapon, but is more to fill a specific niche, rather than being something meant for primary fights.
So, for its XP cost, this weapon can certainly have its place. Unlike other weapons, which tend to fill a secondary role, this weapon is more of a primary pick. If your hand slots are precious, then it's not the best option. However, if you don't need both hand slots and don't need as much of a raw boost, this might be a solid pick.
The main downside regarding low XP weapons remains that most of the time, full fighters prefer to jump to one of the 5-XP weapons to keep their efficiency high. Even non-guardians will have Timeworn Brand to jump to, and Guardians will jump at the opportunity to have their pick of Lightning Gun, Flamethrower, or Holy Spear. Even if there's XP available after, there are times where it's more cost effective to find a Backpack (2) rather than try to purchase more weapons.
So, is this worth getting? It depends on what you need. It's not one of the big guns that will let you take out a boss on its own. If you're looking for something to fill more of a 'backup' role or absolutely need to be able to spend your entire turn attacking, it may be better to look elsewhere. And if your hand slots are needed for other things, this will be awkward to fit in. However, if you have a reasonable to back it up, this is a solid weapon with good staying power that can last for a good chunk of a scenario. If you can take advantage of the bursts that it uses to fight, it's worth consideration.