Personally I prefer to map outside the VTT, so my answer to the query in the title would be: "Not to map in VTT". Anyway ...

1. Which VTT do you use?
Fantasy Grounds
2. Does it have the ability to allow you to map in it?
Yes
3. Do you use that ability?
No
4. How quickly can you produce an encounter map with it?
Very, freehand drawing
5. How is the quality of the map?
As good as the person drawing, so in my case v. bad). It's better with a tablet (or so I've heard)
6. How usable was the map?
Same functionality as any other map in Fantasy Grounds (grid, masking, drawing tools, pointers, tokens).

The rest isn't really relevant as I don't use the functionality. My free hand drawing is terrible. I think I've mentioned my lack of talent before.