The 16:00 check-in time is very strict. Your room may be cleaned, ready, and waiting for you at 15:00, but not allowed check-in until 16:00 (unless you're a points member, then you can check in whenever you want. Honestly that seems to be a very cheap trick). Even 10 minutes to 16:00, was made to wait in the lobby until exactly 16:00.
That one little fact irked me exceedingly. May not for other people, but considering I paid for the room I would like to be able to go in it in the late afternoon after a long travel day.
Check in times are meant as an approximation, not an absolute.