Myself and four others did the Paris to Nice train, we did enjoy it and all went well. I found that the guy strapping the bikes in was much better at it when he saw the ten Euro not I had in my hand.
There are a couple of things you may wish to think about. On our trip, maybe not on all, as said in previous post, everything must be fixed to the bike. Nothing removable. The maximum height for the bike is approx 1.5 metres as the upper deck is very low. We put the bikes on one train which left at about 5pm, we then got the shuttle bus, included, to another Paris Station where we had to wait until 9pm before the Sleeper train departed. We made good use of this as a few of us went into central Paris for dinner and beer. Whilst this may sound great there is a downside. Unless you want to spend all of your waiting time in your bike gear, with helmet and wearing boots etc, you need to allow space to pack it on your bike securely! Two of our guys wanted to wait at the station therefore we were able to leave the kit with them.
When on the train they provide ear plugs etc which I would advise you use as it is a bit noisey whilst sleeping. Another thing is that we arrived at 8am in Nice. The bike train arrived after us and we didn't get the bikes until noon! This was a pain. However, it was a good trip, we enjoyed the train etc. A big bunus we felt was that in Nice the sun was shinning, the sky was blue, we were fresh and the ride was spectacular. I hope this helps. Ah! Take lots of water on the train. They give you a small bottle but it isn't enough. We took some baguettes as well. Enjoy.