I'm liking it! Fun to do the challenges, and fun to browse what others have done.
As for IAP... my coins are increasing on their own, and I haven't found where to spend them! They've kept the IAP maybe a little TOO out of the way.
I'm led to believe you can buy more colors (single-use? forever?) but the default palette has the basics. Maybe you can only spend coins WHILE you're drawing? I'll try to remember to check next time.
(Universal would be neat--I guess they'd have to add pinch zoom to make the small format work.)