Overnight buses? Fuk that! I stick to only daytime ones. Being woken up by a passenger's loudest mobile ringtone known to man every 40 mins does not, and nor will it ever, make me a happy bunny.

The following is just a mish-mash of waffle about some of my bus traveling.

My latest bus trips for the last 2 years have all been to and from Mae Sot. But prior to that I've bussed it a few times to Nong Khai and Ranong.
I've tried several of the bus companies (that cover many other routes in Thailand) and the best one turned out to be the cheapest at around 200b. There was no seat headrest digging into the top of my shoulders (I'm 1.8m) thereby making me slouch and causing my back to ache like that of VIP bus seats that cost 480b. OK, so the VIP bus gave me a 5 baht's worth of Thai cola in a plastic cup and a packet of biscuits worth about the same. But, I can live without them.

The Mae Sot/Tak buses all covered the same route and got me to my destinations at about the same time. Only difference being the choice of rest stops for a meal. Different stops but the food served is equally as pig-swillish as at most other rest stops too. If you're lucky, there may be a 7-11 close-by to the restaurant stop where you can buy something that more resembles food or some nibbly dibblies, munchables, crisps, chocs etc. and none of those OTOP selections at rest stops of dried tadpole tits, dried prunes, dried scambled spiders legs, pork scrappings or scratchings, lime-green sponge cakes etc.

The Nong Khai bus that I got stopped at Khorat (you'll probably stop here for a meal break) and Udon on the way, as well as every other place with a population over 20 or so it seemed. I could've flown back to England in a shorter period of time than it took getting from BKK to NK. So I would advise a VIP or Express bus to Udon if you plan on going directly there. Don't bother asking at the bus station what time a particular bus will arrive at your destination as you'll only have to add or subtract 2 hours from what you're told. Ask the driver to get a more accurate guess. You could break up your journey with a stay at Khorat, an instantly forgettable town unless you live there in which case you won't have that option.

Seating. When booking at the office you'll be shown a seat plan. If available get the 2 seats so that the stairs are directly in front of you. You'll get more leg room and an unobstructed view of the TV placed at the other side at the top of the stairs. And obviously there'll be no-one sitting in front of you who can fully recline their seat into your lap.

All the bus companies over the last 5 years that I've used have had fairly good drivers and pleasant enough crews, and have never had any speeding loonies behind the wheel like they had many years ago. Now the seats and service is almost airline standard.