First of all, Mekong Express is not 'luxury' as my guide book stated. My bus was old, the aircon barely worked, and was by say Thai standards third class. Maybe others are worse, but I saw a bus from line called giantibis that seemed better from outside. On plus side, they do check baggage well with ticket and arraneg tuk tuks at fair price upon arrival.
The worse news. Be careful with your belongings. At rest stop they warn you to take all your belongings off bus. A sure sign of problems given they have bot driver and assistant that could lock and watch bus but dont. Upon arrival it seems that my Ipad fell from my lap to the floor when I fell asleep and I forgot it. Realized as soon as I got to hotel and called, but of course they said no ipad on bus. Equally of course it had to have been there. So heed their warnings abbout making sure you have everything, because if you don't, its gone!!