I just tried out this program and I am quite impressed with how it works.  And it's also FREE.  You can download it from here:

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I had tried out the trial of SketchBook Pro some years ago but gave up on it when I heard Autodesk was going to stop development of the program.  Then Autodesk continued the software and released a new version.  They also made a simpler free version given away with tablets.  I found about this one from a forum.  This is based on the free version but has brushes that emulate the popular Copic markers and pens when used with a tablet.  It works quite well with my Intuous tablet.  It can save layers in the TIFF or Photoshop PSD formats.  It can also save in other image formats but with no layers (it flattens them).  This one is well worth looking into especially if you like working with Copics.