Another quick test using some freeware called "Paint.NET"
Unfortunately (for my sketch style) I found the tools to be somewhat limited. The main limitations I had were with the brush tool. Everything else looks to be in order :)
Image drawn using my wacom and Paint.NET (version 3.5.8)
PROS = supports layers (plugin available for export to PSD), supports some blend modes, layer opacity, image and canvas resizing, most tools that you will find in photoshop, most importantly low cost (100% free to use).
CONS = no pressure sensitivity, some keyboard shortcuts vary to PS defaults which can get confusing, no option for soft/hard brushes, no other brush settings available (other than size), no mask layers, no pen tool, not the nicest UI but it is free after all.
At the end of the day, whether or not you get value out of a tool like this depends on what you are trying to achieve, and how you go about reaching your goal. Any piece of software is merely a tool to help you create your artistic vision. There are MANY tools/features in Paint.NET that would be attractive to people who want to edit photos and make transformations and adjustments to colours (Levels, Hue/Saturation, Brightness/Contrast etc). But as for sketching, I found this tool quite restrictive.
Sometimes limited tools can lead to interesting results. If you can learn to adapt to the tools available to you, you may be suprised by what you come up with... I was.