Well, I managed to join the queue for the restaurant car early by taking the Hammersmith and City Line to Paddington and accessing the platform from the west end.
There were tables laid part way into coach G. There were a few who had reserved a seat in the restaurant, and a few others with standard class tickets also wanting dinner.
They seated the First Class passengers first then found the rest of us a seat. The food was absolutely lovely but a bit pricey - over £30 for two courses, coffee, a beer and mineral water. The staff were having a nightmare clambering over people sprawled throughout the gangway.
I'd recommend the FGW Pullman but caution on price. I heard a story from a fellow passenger that the two evening services that still have a restaurant car were almost abolished but were retained following a lot of pressure from SW businessmen who all wrote in and complained.