Will the Spoaster protect me against all spills? 

Just as a waterproof watch or phone will work to a certain point, so the Spoaster will protect your drink up to a certain force. However, you would be surprised at how many accidental spills are knocked over with just a slight nudge.  

How much more tip resistant does it make my drink? 

This answer varies because there are so many factors which affect this value. It is based on the drink container (height/width/weight/shape) and also the amount of liquid left. For example, a soda can is damn near close to untippable with the Spoaster. One thing I can  promise you is this though : If you managed to spill your drink while in the Spoaster, there is basically no chance it would have been survived without the Spoaster.