We really enjoyed our trip from Bangkok Ekkamai bus terminal to Koh Chang.
The bus driver was really friendly due to the fact we were the only tourists in the bus.
20 min stop after about 2 hours at a small shop were you can buy something to eat/drink.
If you have big luggage (2 backpacks like me e.g.) please take 2 seats for each person, the bus doesn't have a trunk.
We got out of the bus after about 5h at a small building near to Trat were they brought us to the Koh Chang Ferry. (30min Sharetaxi + Ferry 180 Baht per person)
All in all I can really recommend that kind of trip!
Easiest booking experience ever! Made my booking in a couple minutes - no need to go to the bus station early. Also, booking early allowed me to choose and get my preferred seats. (Tip: in the 20 passenger minibuses 2C and 2D have the most legroom. Bus tix are cheap so I usually buy both - but I'm not on a budget
Ride quality is ok - not great...no recline and fairly hard seats but for 3-4 hour rides, I'm ok. I'd opt for a full tour bus for longer sojourns.
The driver is safe - never over about 90kmh and they actually post a live feed of his speed for everyone to see.
I speak Thai so it's all really easy but non-speakers could have minor challenges - though all the employees do try their best when I see tourists interacting with the staff
When i arrived there were plenty of busses leaving, I wish I hadn't booked with you because I had to wait for a long time and the bus was small and not comfortable and we arrived very late. Next time I won't book, I'll go straight to the station and buy a ticket there. With your site, I couldn't book for an earlier bus, I don't know why. So, it was not so good.