Using TGV for three trips recently in France we found a wide range of customer service between the stations in Paris, Dijon, Lyon and Marseilles. Sometimes there are helpful staff to guide you to the right car and sometimes not. Sometimes they scan your ticket and sometimes you ride all the way to your destination without it being checked.
We indulged in premier class and anticipated using lounges - not every station has one. Take off your sunglasses as they will make seeing the illuminated signs on the side of the car easier to read and avoid the confusion we experienced not once, but twice. (You'd think we could have figured that out the first time.) Seat assignments on TGV trains are also tricky - don't look overhead as you would on a plane. Lean slightly over the passengers already seated to see the numbers inconveniently located between the seats.
Most of all - learn from our biggest mistake. Doors close and are locked at 2 mins before departure. Yes, if your travel to the station has more than a few hiccups, even if you run all the way and the train is sitting still on the track, you can not board. Furthermore, you can not get a refund and will have to pay for another ticket to get to your next destination.