I was on the 09:20 Barcelona to Paris TGV Duplex the day before yesterday - only its 2nd day of operation. Spanish TV news crews were everywhere...
Very smooth, very fast, and into Paris 3 minutes early.
The easiest place to buy tickets is at www.capitainetrain.com or www.loco2.com (which can also book you through to or from London or any British station) as the official French Railways website has an annoying tendency to divert you to rail Europe, where there's no seating choice. I strongly recommend an upper deck seat for the best views.
Fares start at €59 in 2nd class, €99 in 1st class. You normally simply print your own ticket.
There's no WiFi, but the train has power sockets at all seats, a cafe-bar, toilets & luggage space both upstairs and downstairs. You're free to bring your own food, wine or beer on board, or you can buy from the bar car.
It's quite a scenic run - past snow-capped 2,784m high Mt Canigou at the southern end of the Pyrenees (which was visible from just before Girona all the way to Perpignan), past the 15th century Fort de Salses just east of Perpignan, along the Mediterranean coast, past many étangs (lakes) with colonies of flamingos feeding on one leg in the shallows, over the River Orb then right past Béziers' hilltop cathedral, then up the Rhone Valley past pretty villages and even the odd castle. It was a very clear day on Monday so I could see the high French Alps far to the east as well as the dark ridge of the Massif central to the west.
I then hopped on the 18:13 Eurostar back to London.
Subject to confirmation, they plan to double Paris-Barcelona TGV service to 4 trains a day in each direction from 31 March 2014.