I use the Pullman dining most weeks either to or from Plymouth (35 trips so far this year...!). It's a fantastic service on the whole.
The website can help you figure out how busy it will be, but even so sometimes it can be unexpectedly full e.g a random Tuesday in February. 2 weeks later I was just 1 of 3 people in the carriage. I've certainly seen standard class ticket holders turned away, but more often than not they have a seat found for them. Higher chance of being given a seat at one of the tables for 4 as they normally hold the singles back for people who book. Friday night down is the toughest gig as there is a regulars club who book it up!
The current post June menu is solid. The starters are better than the previous version (the ham hock terrine is excellent), the mains a bit less good as the sage chicken isn't a patch on the previous version with blackberry sauce. Eating bouillabaisse on a train means lots of splash on your shirt front... Don't do it. The food is genuinely good, I'd happily pay for it in a normal restaurant let alone rolling through the countryside at 100mph.
Crews are normally excellent. There are a couple from Plymouth who could do with a bit of charm training but most are really sweet. There is one London-based service lead who I almost always see at Plymouth and she gives me a wave and ensures I get my favourite seat.