Im with Antonio, Id love a Soundplane to complement an Eigenharp.
But, I don't really think they are comparable, I think they would be great for different purposes.
I don't think the soundplane would be comfortable for playing conventional pieces
(unlike the eigenharp tau/alpha which is great in this regard)
a) its not a soft surface, it has limited give.. this i think will not be ideal for playing lots of notes, say like a piano
b) although its a good size surface, there are limitations with finger spacing, and I suspect that it might be difficult to find a general purpose layout with a good range, that allows 2 handed playing with chords. (the eigenharp has shown me, despite lots of keys, finding a good working layout is tricky)
If I were considering it as a main instrument, i would mock up a layout (dimensions are on madronas website) put notes on it, and see if you can play what you would like.. in particular pay attention to number of octaves, and playing triads using a chromatic scale, you could also put a small piece of rubber/foam (a few mills thick) under it, and see how it feels to play on a semi hard surface.
of course, if your not planning on using it in the same way as a conventional keyboard, it may be fine.
On the flip side, I can really imagine molding sounds, and sliding around the surface on a soundplane, which could be 'zoned' for all sorts of different purposes - I think it would be fantastic for this.
(this could be done to some extent on an eigenharp, but not in quite the same way)
I can also imagine having some fun on less conventional pieces, or for just 'noddlng' around, trying out ideas... it would be great to have in front of the computer and just slide it forward to play.
in this regard, I think it would be similar to how I still use my pico... ie. i just grab it when I'm not sitting down to 'actively' practice/play, I'm just messing about... perhaps playing with a VST or something
doh... i wish you hadn't brought this up... I don't need more temptations to get a soundplane :)