I was all set to buy this but during the 100 or so minutes I've spent with it so far I've been hit with the same bug around 8 times.
A cluster of 10-12 pixels (probably larger than individual pixels but squares at least) will just disappear close by while using a brush, it's happened on the organic and the calligraphic brush so it's not just one of them, usually I think when I'm moving the cursor quickly sketching. I have saved a PNG still showing the issue so it isn't purely a screen rendering issue.
Undoing doesn't return the pixels to that part of the picture is effectively ruined and needs to be re-painted.
I know it's a beta and I accept that as such there are a limited amount of tools but having it punch holes in my picture while I'm working isn't really something I can live with.
My PC is easily up to the spec (i5 3450 and HD7950 with 8Gb ram) and it's nice and fluid in use and a great idea particularly for rapidly producing concept type art without having to import lots of texture but not something I could put money on at the moment.
How long will the beta offer be available? I'd be interested enough to check back when it's more stable but only at the discounted price as I already have other software I use (SAI and an aging but still useful Photoshop CS2).