I've used Five Star Service when travelling with my less savvy parents and in-laws on family trips. They are more used to the old days of flying, and not the hoops that we jump through today, and with Five Star we've received excellent treatment and been able to avoid quite a bit of hassle, especially at LAX and LHR.
Check-in isn't anything really special beyond a seperate enclosed area to make us feel special or for celebrities to hide from the paparazzi. It feeds into the same Pre-check/Elite line as the main check-in lobby for elites.
However, Five Star Service is excellent when it can be arranged. I especially appreciate the choice of either being the first on the plane (pre-board) or last to board (gives more time in the lounge). We've also made good use of the laison with car service, especially on inbound flights (proactive on notifying driver of IRROPs, etc.)
This should all be qualified with Five Star is not always available, and we've been fortunate to have excellent Premium Services staff (as always staff can be a variable).