Both mentioned above are very good but as also mentioned, the learning curve can become a stumbling block. They offer the convenience of being able to paint on your model directly in real-time depending on the speed of your computer. Ive used both in their earlier versions and they do take some getting use to. Ive also used a few others, some now defunct (MetaCreations Detailer became DeepPaint 3D and I think 4DPaint is no longer in use or evolved). I find the use of a model unwrapper like those used in 3D StudioMax or UVMapper and bringing them into my favorite image editor (Photoshop, Paintshop, Dogwaffle or Painter) and arranging the skin how I want it very convenient. For Photoshop users, you can create an Action micro that allows you to use Photoshop while viewing the results in UVMapper.