Traveled with my young daughter and decided to take the overnight train from Venice to Paris. First, you need to manage your expectations. It's not a high end luxury train but generally everything is very good. Though we had 2 people, we booked a private 3 cabin sleeper to make sure we didn't have any unexpected guest - booked directly on Thello site. The room was clean with a small sink basin that included some slippers, toothbrush, and mouthwash. There's no TV and wifi (though I did hear some trains do have wifi). We traveled in April and it was a very quiet train and not full. Food was ok but a bit expensive. Though it was neat just being able to dine in one of the cars - they do take credit cards. You can also purchase a few things a la carte or just bring your own food, drinks, sandwiches, etc. With the sleeper tix, you do get a small continental type of breakfast (coffee & danish) but make sure you don't go at the end when they start to wrap things up.
They key reason I'd use Thello for this same trip again is the convenience. We stayed less than a 2 minute walk from Santa Lucia termini so we didn't have to pull our luggage across Venice. Plus all you need to do is literally get there 2 minutes before the train takes off. They handle customs/passport during the night. Much better than taking the Chunnel where you have to be there 1 hour before departure. This was an overnight train so there's not a lot to see as it is dark - summer might be better. Did have a few incidents late a night with people trying to board the train, but the attendants were totally on top of it and still felt safe in the end.
You figure instead of paying for a flight, going to Venice airport, taxi, and additional room night in Venice or Paris, your cost on the Thello is a bargain. You board at 8pm and arrive in Paris at Gare de Lyon at 9am, and from there can catch the Metro to anywhere in Paris. The ride is a bit rough at times, not quite as smooth as the TGV or similar overnight trains in China.
Overall a great experience. A great resource is the website seat61.com, for all train travel info in Europe.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.