Just a couple of clarifications on this discussion:
The older model Sidewinders would not take certain Viking and Bernina bobbins.
The later models do pretty much all of them. This is explained on the box, so if you go to buy one, make sure you're getting a model that'll work with your machine's bobbins.
Later ones also have a 110 adapter, so you can use it with wall current and don't have to use batteries.