I often have days when I can't believe how horrible my writing is. Sometimes it's a sign that there's something else wrong with what I'm trying (typically viewpoint). Sometimes it just means I've gotten too deep into the nuts and bolts that I've lost track of what the reader is feeling.

OTOH, there are days when I'll be reading something else and suddenly writing will seem so easy. "I can do this!" I think. "This is what we need more of!" (And also times when I might read a book and think "This is rubbish! I could do much better than this...")