I've been playing around with it for about a week now, and still can't find a brush/mixer/finger setting that will let me blend as well as I can in Sai. The mixing tools all seem to pick up clear areas, which in sai, the mixing brush's ignore clear, and will only pick up color, and fill the brush. This is a major annoyance to me. Photoshop is the same way, and I really can't stand blending in Photoshop because of this. In all other ways it's very superior, except I'm having a cursor alignment problem with my widescreen tabletpcs which Susumu is trying to help me with, but not succeeding so far.
I successfully purchased the program. Susumu emailed the online store and then the store emailed me, and I emailed them my personal info, which they then set up a foreign account for me. Then I was able to purchase the program with a card..so if anyone is interested and doesn't mind dealing with no English support, it's definitely a superior program. As much as I love sai, I've been working in it for like 4 years now, 4paint beats it in almost every area at this point. The major factor is that it allows for much larger canvas's and paintings. In sai I'm limited to about 4000x3000, before my 4gig memory system starts throwing errors at me, with 4paint I can go up to 8000x6000 easy.
It's somewhat less user friendly with selection and modification/rotation, than Sai is, but again the difference is minor enough that I would still prefer 4paint since 4paint allows me to double the resolution I'm working with.