The bus trip from Chiang Mai to Bangkok is one of the most used travel routes in Thailand. The journey is 360 miles as the bird flies. Naturally roads, even Roman ones, don't dissect maps in straight lines. However, if you were to draw a straight line from Chiang Mai to Bangkok and overlaid the actual road route you would see the two are fairly close. Indeed by road the journey is 425 miles.
In a car the trip can be done in just under 9 hours. By bus the journey takes 11 hours. The bus doesn't stop all the time to pick up and drop off passengers. There is usually one stop of 40 minutes or so to get food, stretch the legs and go to the toilet.
There are a number of bus companies that cover the bus route from Chiang Mai to Bangkok. They all have air-con and toilets at the back of the bus. Many of them have video players that show movies during the trip, except at night.
For the backpacker, travelling by night has 2 major advantages. The first is that you save a night's accommodation cost. The ticket costs about 600 Thai Baht. The cost of a cheap room in Bangkok is about the same. Another plus is that you arrive at 7 am. You have all day to find your room in Bangkok.
The disadvantages are that you cannot see any scenery travelling at night. You miss the Thai countryside. Also you cannot lie down in the bus. The seats recline a bit but as you go back so the person in front of you does the same and you feel just as confined as before. It is hard to sleep sitting up. Moreover, the air-con cannot be turned down. It is essential to bring a blanket or a sweater as you get cold at night.
It might be best to try both day travel and night travel in Thailand to see which you prefer. I find the experience of travelling such a long distance tends to jade me for several hours after disembarking at the destination.