This plugin is the "Zebralette" of the piano market. When companies produce these cut down freebie versions of their products we all win. The company gets goodwill and familiarity with it's products, and we get free stuff!
I've been using Piano One for all my practicing, improv and composition needs for the best part of a year, and the sound is on the whole very satisfying. Now, you can practice and do bare-bones composition on polysynth sounds, but for some reason, you just can't beat the experience of sitting at a piano when it comes to teasing ideas out of your subconscious.
This piano is even good enough to be used on professional tracks semi-naked, with one caveat - the release - as has been pointed out in the other review, is pretty unrealistic. You can fix it a bit by dragging up the release slider, giving a more realistic release length, but the *shape* is still wrong. Mostly you can fix this with reverb (using the reverb that comes in the plugin or an external one), but still, it's the fly in the ointment of an otherwise near-perfect freebie, and the reason I'm giving 9 instead of 10.
The look of the plugin is pretty janky, but the interface has some good controls (velocity curves for example, very useful for extracting different piano styles from your playing). Their paid plugins have much nicer atmospheres to them and it seems fair that they'd cut back on this a bit in the free version.
I'll definitely be checking out their other piano products with the hopes of buying them if they can build on the quality of this one.