Transitioning from differently lit rooms/areas can be a bit difficult to adjust to for a few minutes. Also hard to see at night. The key is to follow Dylan's s4 protocol and you can avoid the vision sides as much as possible.
As far as the long term, I'm on my 5th run of s4 over the last few years and my visits to the optometrist have been excellent. Zero changes in my vision. The sides stop immediately after discontinuing s4.