I downloaded Sai and was very impressed with it, (unfortunately I only had a couple of hours play with it before we left for holiday and I didn't bring my tablet with me. I'm looking forward to playing with it a bit more on my return. Serif Drawplus is my main tool at the moment (it has for me about the right compromise between raster and vector functionality) but Sai really did impress. I'm using photoshop less and less these days for mapping.