Mistakenly rushed out and bought a rubi washboy soon after finishing my tiling course 2 yrs ago. Used a karl dahm one on the course, and I can't wait till my rubi one finally gets retired.
Still, I always have 2 or 3 grout floats for different shades, and only use segmented sponges. Trick is to try and get as much water out of the sponge,
Have used on walls and floors, but will chop and change between a bucket/sponge and the washboy or even both depending on the job. Nice to have the bucket of fresh water on hand anyway.
Started to try out the little detail sponges from tradetiler and they worked a treat for edges and getting in behind things like exisiting pipework. The sponge soon came off, so I plan on ordering a replacement.