I went for a couple weeks during the honeymoon.
It was the best time I've ever had.
Bangkok is crowded, dirty, and the air smells horribly from all of the motorcycles. We spent a few days there before taking an overnight train to Koh Samui. That's where the real fun began. Everyone says that taking a first class overnight train across the country is a must, but I would not do it again. With flights all over the country so fantastically cheap, we flew everywhere the rest of the trip.
Spent almost a week on Samui. Diving, wandering, lots of drinking. The diving was OK; not as good as Honduras, but did see a whale shark.
Make sure to drink plenty of Thai beer.
From Samui, flew to Chiang Mai. Awesome. Great food, beautiful temples. Elephant sanctuaries are nearby. Took a hot-air balloon over the countryside and some temples.
We visited the Siam Thai Rice Cookery School
http://www.siamricethaicookery.com/ for a half day of cooking classes. It was amazing, and I've never eaten so much or so well.
From Chaing Mai, spent a few days in Tokyo before heading back home.