I have now completed my southern coach trips which were fun, interesting and aggravating in equal measure. My route was Hat Yai - Samui - Hua Hin -Phuket. Following advice I decided to opt for the government coaches and ticket purchase at Hat Yai was simple with choice of two morning departures at 08:00 and 10:30. At time of purchase I saw a beautiful VIP coach leaving for Samui that day and expected same. Then the fun began. Arrived at station for my departure a few days later to learn there were not enough passengers (there were over 40!) so the two VIP coaches were cancelled and would be replaced by a single service leaving 90 minutes late and arriving Samui two hours late. In practice it was an old second class wreck but did have air conditioning. The real reason for the change in service soon became apparent; this was a truck and not a bus! We stopped every few miles along the road and all manner of farm produce and other goods were loaded onto the bus and money changed hands with the driver and his assistants. I learned from one Thai woman that this is common on this route and if there are only a few passengers then they will cancel service and put them on a minivan. It seems you pay for VIP but should expect anything. All things considered the stops along the route etc kept my interest and the journey wasn't too bad.
From Samui to Hua Hin by accident rather than design I ended up with a private coach from an agent rather than a government coach. The old dear at the agency skillfully extracted a few more baht for the ticket than I'm sure she should but I didn't mind as she was selling me schedule I wanted which I might have missed elsewhere. This coach was going from Samui someplace north of Bangkok and was very clean and comfortable with seats akin to aircraft old-style C. The seats had substantial recline, footrest and massage function. Lavatory on the coach was very clean. Snack and drinks were frequently served. The journey was comfortable, pleasant and uneventful but long. A word of warning; the agent will inform you that some of these buses going north of Bangkok stop in Bangkok too and try to sell you a ticket on them. In practice they stop near Bangkok and you will likely need to arrange your own taxi or minivan transfer into Bangkok by yourself regardless of what ticket agent tells you. So, if going to Bangkok it is probably best to choose a bus where that is the final destination.
The final leg of my journey was from Hua Hin to Phuket on a government VIP bus which was similar to but not as new and nice as the one from Samui to Hua Hin. The bus was only 15 minutes late in departing Hua Hin despite arriving from Bangkok but it made up time and arrived 15 minutes early to Phuket. There were quite a few tourists on this service and the hostess spoke good English. She was happy to play DVDs belonging to passengers at their request provide they were English with Thai subtitles or vice versa. The scenery on the last few hours of this journey into Phuket was very enjoyable.
On the whole I enjoyed my coach trip experience as it was something I always wanted to do. Despite this I will be unlikely to use the coach again unless there are no flights to my destination or for some reason we do not want to drive. When using the coach I'd advise patience and be prepared for the unexpected. It is also a good idea to take plenty of food and drink onboard of your own liking as journeys are long but with usually only one rest stop. Finally, if cost is the main factor in opting for coach travel then I'd advise checking out Air Asia first. For example, they frequently have special offers from Bangkok to Hat Yai, Phuket etc which can be substantially less expensive than a VIP coach.