Several private companies offer quite comfortable minibus service from either Bangkok or Pattaya and it is, in my opinion, probably the best mode of transportation to the island in terms of relative comfort vs. price vs. travel time. Minibuses are ready made to transport about 12 people and trip from Bangkok to Koh Chang takes about 5 hours incl. ferry. Minibuses depart from Bangkok mostly from Kao San Road, where you can also purchase tickets in any local travel agent. From Pattaya to Koh Chang minibus service can be purchased in virtually every travel agency in Pattaya. Service provided by such well-known company as Pattaya Group and few others. Minibuses from Bangkok and Pattaya have daily departures at several different times.
Tickets from Bangkok to Koh Chang can be bought at the price about 650 Baht and from Pattaya then about 550 Baht.
In addition to regular minibus lines you can rent the whole minibus for yourself, if you want to have more privacy or traveling in larger group of 5 or more persons. As the reference price for such charter, you should not pay more then 5.000 – 5.500 Baht from Bangkok including ferry and from Pattaya it would be approx. 1.000 Baht less.