They offer some protection and will trip for appliances or humans etc. but aren't as good as when they are grounded and don't control current well enough for electronics etc. Still much better than trusting the circuit breaker to stop your kid getting shocked to hell.
We are about to have the grounding fight with the outlets in our new home it's all weird because a single wall will have two 3-prongs that are correctly grounded and then one 2 prong. I'm hoping that the boxes are grounded somehow but I'm guessing it's going to cost a bit to get everything rewired...