I use my sketch books in my studio all the time, in fact I usually have at least 2 or 3 in use of different sizes.

The recent series I just finished "Chakra Drawings", I used them a lot, often its to clarify what to do next.

I usually won't plan out a drawing with a sketch first, rather I will draw in my sketchbook when the large drawing has gotten out of hand and needs clarifying.

Here is an example , it is the root chakra.