Space bags or as they are also called Compression sacs can save up to 70% of vertical space inside your bag if you are packing for instance a fluffy down jacket. But if you are using a ski jacket then the compression will hardly be anything. In general tough I do not recommend packing your jacket since the jacket contain pockets you can store stuff in and also you are typically allowed to carry a coat/jacket in addition to your luggage on board a plane so you can store the jacket using something like a Travelon bag bungee or add a bag strap and keep the jacket on the outside of your luggage for convenience and space saving.
As for other compressible items such as sweaters, socks etc then I recommend the use of eagle creek compression sacs:
http://shop.eaglecreek.com/packit-co...l/d/1065_cl_-1
When packing compression sacs always pack them last on top since they are rather large and although they save vertical space you lose some horisontal surface area in the bag. Another disadvantage to the use of compression sacs is that they become rather hard so if you carry delicate items inside your carry on they will be less protected since these are hard compared to non compressed soft clothes that would otherwise have offered more dampening. Another disadvantage is if one of them breaks then you might find yourself in a situation where you can not close your carry on. So it is always recommended to carry an empty spare compression sac for safety!
Another thing worth mentioning is that compression sacs are not packing cubes, meaning not made to last years of use. Do not expect more then about 10 flights/5 round trips or so then you need new ones. So they are disposable in nature. Always squeeze out the air very slowly when using them to not put to much stress on the exhaust valve at the bottom.
I hope you found my advice usable in some way!
