The winter event taking place in 2017 has added a new item, the Star Piece. Like the Lucky Egg, this item will boost your Stardust (instead of your experience) by 50% for 30 minutes.
Players have had issues with gaining Stardust for a good while now. Although these Star Pieces can only be found in premium "gift boxes", their addition has been very welcome. But what do the numbers show?
Here are the boosted Stardust rates:
|Stardust||Source||Stardust with Star Piece Boost||Stardust with Star Piece Boost and Double Stardust Event|
|20||Feed Berry At Gym||30||60|
|100/300/500||Catch 1st/2nd/3rd Evolution of Pokemon(babies excluded)||150,450,750||300/900/1500|
|25/75/125||Weather Boost for 1st/2nd/3rd Evolution of a Pokemon (babies excluded)||37,112,187||75/225/375|
|600||Regular Daily Bonus Catch||900||1800|
|3000||7 Day Streak Catch||4500||9000|
|400-800||2km Egg (about 300 dust/km/egg)||600-1200||1200-2400|
|800-1600||5km Egg (about 240 dust/km/egg)||1200-2400||2400-4800|
|1600-3200||10km Egg (about 240 dust/km/egg)||2400-4800||4800-9600|
|500||Raid Battle Completion||750||1500|
What we can extract from this is the Star Piece item will affect just about every source of Stardust in the regular game. Star Pieces affecting AR+ is still being tested. It can also be speculated that this will stack with typical double Stardust events, which would result in a total bonus of 3x Stardust.
Reddit user u/str8rush broke down what a player stands to gain from the full potential of a Star Piece. You can find the math at the link above, but the conclusion he came to is as follows:
To sum it up, I came to the conclusion that the Star Piece can be enormous valuable under certain conditions. If you manage to hatch 9 10 km eggs and afterwards catch as many pokemon as possible at places like Centro Oberhausen or other Unibail-Malls, it is possible to get roundabout 50k SD in 30 minutes.
Imagine that during a Double SD Event! Taking into account that you stay at these malls for at least a few hours, it is to say that even 50% are a really nice boost for those days.
Otherwise, the Star Pieces seem more like a bag filler to me.
TL, DR: Star Piece is not that worthy at all, only under special conditions.
After the winter event in 2017/2018, Star Pieces became unobtainable. Hopefully they will make a return sometime, but for now, it seems like not much was missed.