Oh I almost forgot...... unless you stay somewhere half way (Pattaya, Jomtien or Ban Phe area) going to and from Koh Chang, it's a L........O........N........G trip by road.
We stayed at Novotel Ban Phe for a couple of days to break the journey, but on the return trip we went from our hotel on Koh Chang back to Bangkok - it was almost 7 hours (with only 15 minutes total breaktime).

If I went back again, I'd definitely fly at least one way. Prices for a flight or for car/taxi are pretty similar.