I've never tried one but I fancy getting one!, however a recent 3d world review reckoned Wacom has lost the bang for buck competition to the genius pen sketch....

Here's a thread from the 3dworld forum:-

http://forum.3dworldmag.com/viewtopic.php?t=9558&start=0

With a sample vendor linked from that thread

http://www.cclonline.com/product-info.asp?product_id=12629&category_id=384

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.