It's not just the 15 bobbins that are bad. Singer branded class 66 bobbins sold at Walmart are garbage too. They're visibly narrower at the rim than vintage class 66 bobbins and they tend to be inconsistent too. Quality control is nearly non-existent on them I think. I haven't had to buy any new class 15 bobbins yet so can't say whether I've had any problems with those ones.
I hold onto my vintage bobbins. Usually a visual inspection for rough spots or bent ones is all it takes.
Rodney