Some insulae had a multi seated toilet, but it was rather uncommon. It's actually striking how few toilets we find in Roman cities overall. One reason might be that a public/multi seated toilet wasn't to healthy - the Romans used a (shared!) sponge on a stick instead of toilet paper. It's interesting to see that many such toilets were equipped with altars, allowing the visitors to pray before they using the facilities.
On open sewers:
Some cities were equipped with extensive underground sewers but it's a fair guess that most used the "street sewer" system where the high walkways allowed the waste to be gathered and flushed out. We also know that small watercourses were used as sewers (one example can be found in Plin. t.y. epist. X.98).
However, the sewer system is highly complex and I would not like to say too much about it right now, as I have no time to do such in a proper manner.
This should apply to upper class people, I can't imagine that the plebs such luxuries as silver pots, feet washing and daily bathing. On the matter of bathing, I am, and have been for some time, of the opinion that we over estimate how often the common romans bathed (based on number of baths in the cities, how large the baths are and how much time a poor person could spend on leisure. (Rome itself may of course be an exception)
There are actually examples of both these phenomena in Ostia, the first ones that come to mind are the so called garden houses and the Serapis Insula. It is also clear that, even as it is true that most insulae didn't have running water, they commonly had build in drains (you can see this is in almost any roman ruin).
A very valid point Sylla! This is something that people tend to forget when they're talking about roman bathing.