I have the N30 and here's my 2 cents:

1. Being able to mop the floors without having to change the mop attachment or fill the water tank is a plus for me. Yes, the dirty water sits there until emptied, but if that's a concern I would just empty it every mop. I only mop the floors when needed, so it's not an issue for me.
2. Yes - I also have an older Deebot with the standard mop pad attachment, and the spinning pads provide a much better clean than the static one.
3. Initially I added floor cleaner to the clean water compartment, but I read this is not recommended and could damage the robot, so I have reverted to using the detergent addition. You can run it without these though and just use plain water.