There are spokeshaves and then there are spokeshaves. Its pretty much all been said, but quality of the tool a veritas or lie Nielsen are made with thicker blades and finer tolerances(cost a lot more than a vintage or Stanley etc). Some of the older mass produced spokeshaves are really only good for straight grained pine.
You need to know how to sharpen the blade.
Upside down blade trick sounds good, as it increases the angle of blade making it less likely cause to tear out. I haven't tried it with a spokeshave but have used it many times successfully with handplanes.
That being said spokeshaves work best if the wood is already straight grained, they aren't much use on knotty, character wood.