I've only bought a couple of kits. One looked really pretty, but now that it's in the works, I don't care for it. There are too many prints of similar value and together it looks boring. The other has a couple of fabrics I don't really like, but maybe they will look ok when it's done.
I prefer to select my own fabrics. I like to get a variety of prints/marbles/tonals so that there is more visual interest. I haven't really priced it out, but I would guess a kit is a bit more money. There's not much room for error, so if I buy another, I'll buy a few FQs to match, just in case.
I do not prewash kits or precut fabrics, but I do prewash everything else.