The restricted view gets worse towards the sides. The other thing I've noticed that puts me off the upper circle nowadays is that some actors are not so good at performing to the whole theatre (the current cast seem better at cheating up to perform to the circles). I've never sat further forward than row C of the gods, so have never noticed the lighting rig personally.
This is a guide I used to use for figuring out where to sit at the Queens:http://www.theatremonkey.com/QUEENSbooking.htm#Th
You can also get an idea of what the view is like from a particular area using the little guide on this page: http://www.lesmis.com/uk/tickets/booking-information/
You also asked about cast change - that's mid June, I think the 13th this year. Nobody's been announced for the new cast yet as it's still early days, but the current cast is pretty consistent and doesn't have anyone really poor in it. The understudies that I've seen are all pretty solid too. I went to the final two performances pre-cast change two years ago and those were super awesome, though might be more difficult to get good seats for as people who know about such things like to go to those ones.