The PhET simulations are an interesting attempt to all students to experience science as a scientist rather than as a student. This is also done while reducing the financial investments by the school and instructor. They do not however, reduce the amount of preparation by an instructor as they need to know of all the parts of that simulation, so that they can answer any questions that come up while students work through the simulation. However, they can reduce the amount of lesson planning if the instructor shooses one of the lessons from their website. This would in theory reduce as they repeat the simulations, while that is less true for repeated labs. As such, I believe that these simulations have a place to explore ideas which cannot be performed or seen in a schoolroom lab.
Their activities come setup for inquiry based learning, where students can ask their own questions about the particular simulation. For what its worth, I do remember doing simulations like these as a high school student and I did not find these simulations engaging since they were presented as work, and I did not freely choose the topic. Free inquiry based learning to me, requires self-motivated engagement on the part of the students. As such, I don’t know if these simulations can be used as free inquiry, but they could support guided inquiry as discussed by Trevor Mackenzie. This guided inquiry, where students explore a simulation based on prompts by the teacher allows students to explore specific ideas without having the answers presented to them.
In summary, I believe that the PhET simulations have usefulness in classrooms, to engage students and provide accessibility to students who could not otherwise see these experiments. However, they do not provide the level of guided inquiry which or involvement which I was hoping to see.