You don't need to have watched series one through six in order to enjoy series seven. Each episode is a stand alone show and they do not need to be watched in order to follow along. Although there have been some cast changes over the years, DSI Boyd, Dr. Foley, and Detective Jordan remain constant. The cold case squad all seem to work well together and the characters are completely believable. The plots are complicated enough to keep my interest without being so complicated that I cannot follow along. Boyd's son, who has been missing for seven years, returns during series seven which adds some tension to the series.
Some of the subject matter may be disturbing which is par for the course in many crime dramas. I found "Wounds" to be the most fascinating of all the episodes in series seven. This episode involves a ritualistic killing dating from 1995. The investigation involves ghosts and Navajo rituals and even has Dr. Lockhart building a Navajo sweat lodge from scratch.
If you're looking for something in the crime genre but different to watch from all the variations of CSI on television, I definitely recommend Waking the Dead - Season Seven.