Those old Spokeshaves were excellent and I have bought and used many of them over the years. The only problem with the ones you buy second hands is that they tend to be worn out – either the wood or the blade, though sometimes you find a good one. Some of the smaller one are beautifully made from box and are a joy to use.
The problem with many old shaves of this type is that the blade dimensions were never really standardised, so it is difficult to match a blade from one tool to the yoke of another. So it is sometimes necessary to make a new yoke matched to a particularly good old blade. The old article from the Woodworker is a good description of how to make one.
Bristol Design does supply some blades but they are big and rather clumpy and making the Spokeshave yoke itself is a fiddle.
If you are a chair maker, Bristol Design used to do a range of Travisher blades in 6 sweeps and these are worth getting because they are almost impossible to obtain other wise.
The main advantage for this type of shave is the ease with which they can be adjusted once you are practised in their use and the fact that you can keep two settings on the same tool by opening the throat aperture wider at one end to provide a more aggressive cut, finishing with the finer end.
In comparison to the metal type it is a different tool altogether because the angle of cut is much lower, almost horizontal. Allowing for the back bevel and a slight angle of attack below the face of the blade, your compound cutting angle can be as low as 10 degrees. This means that you can cut end grains and grain on contours that are presented at an angle without the risk of tear out or chatter with a superior finish.
Hope this helps – I love old tools because of the simplicity of many of them and in many case they will provide the finish I prefer – straight from the blade.