I have the top 3 (TransUnion, Experian, Equifax) frozen. In my state it was free, no fee involved. You do have to pay to have it unlocked though, but I haven't needed to since I did it about 3 years ago.
I love it most because it has 100% eliminated credit offers in the mail and by phone. I don't carry cc debt so any offers for better rates, ect. are moot, all it means is less junk mail for me and less chance of someone getting a hold of that and applying in my name (I think mailbox theft is less common now though then the days before internet).
The freeze only applies to new inquiries or attempts to open credit. Any existing creditors still have access, so it won't affect your current accounts as far as reviews/increases/% rate adjustments/insurance, etc.
ETA: If you go to the websites of the credit agencies, you can find instructions on how to freeze. I did mine by certified mail.