It keeps getting better, too! The new update makes the S5 basically the same as the newer model (except that it's louder)... I had the original RoboRock and it worked really well. We replaced the battery and gave it to my wife's grandmother, then bought the S5 thinking the mop feature would be in some way useful, but I'm still happy with how well the S5 tracks its brush/filter lifespan and the software upgrades!
I like the mop feature. It is far from a true mopping, but still the floor feels less dusty than after just using the vacuum alone. That said, it may be a placebo effect on my part. I should do a blind test someday.
The problem with the mop feature is that it completely defeats the purpose of having an unattended robot keep your whole house clean, since you have to constantly refill the water reservoir, and it can't transition between carpet and hard floors without getting water on your carpet.