You will have to travel from BKK to Surat Thani (east) and then get a 4-5 hour bus trip to phuket.
If you have never done it before, and you have time, do it - it is worth the experience - but only if you go daytime and want to look out the window at life going by. I cannot see a reason to go at night other than to save some money over flying.
Cheers
