Inkscape: Exploring Spiro
Now you have your development version of Inkscape running, let’s have a look at the Spiro option. Select the Freehand (Pencil) Tool. You’ll notice an addition to the toolbar: 
Make sure the “Create Spiro Path” mode is selected as shown. This will correct your freehand lines to smooth curves which you can adjust using the Smoothing Slider. Currently this can only be set before drawing your line so some experimentation will probably be required to get the amount of smoothing right.
This image shows you how it works. The blue path shows the line I drew. The purple line shows the path with Spiro applied.
As with any path, you can edit each node and the Spiro path will adjust accordingly. 
Shape
Spiro paths can be shaped. This shows each type of path; None, Triangle In, Triangle Out, Ellipse, From Clipboard.
None: This draws a simple line with no shaping which you can thicken or thin using the Stroke Style dialogue. There you can choose the cap or even add markers to your line. Change the colour using the Stroke Colour dialogue.
Triangle In & Triangle out: The line is thickened at one end. By default you can change its colour using the Fill Dialogue but you can also apply a stroke colour. If you do so the stroke is an outline of the triangle shape so thickening it will eventually cover over the fill colour rather than increase the size of the whole object.
From Clipboard: Use an existing path to shape your spiro path. In the example above I used a star shape and applied it to a curved line to create the deformed star shape. You must use a path for this trick to work. If you draw a shape you’ll need to convert it to a path first Path>Object to Path. The colour of your original shape will not be preserved.
How can you thicken a shaped Spiro path?
Try resizing a shaped Spiro path and you’ll soon see that proportionately remains the same, but what if you want a much thicker object than the one you drew? If you thicken the stroke you’ll find the lines are blunted and the fill colour becomes occluded by the stroke. The secret is to draw small and scale up. Here’s an example.
The two paths on the left show how scaling a medium sized path up slightly thickens the overall object. The two paths on the right show how drawing a very small path allows much more noticeable thickening when the image is scaled up.
Notice how the stroke has scaled with the image? If you don’t like this effect you can decrease the stroke thickness through the Stroke Style Dialogue.
This is quite a dirty workaround to the problem and hopefully a better solution will be incorporated into the final release.