You can't take your car to Samet.
You wouldn't want to drive your car on Samet - there's really just one road running the length of the island, with a few minor roads leading down to the various resorts/beaches. They're all lumpy-bumpy dirt roads.
To get around you can hire motor bikes or quads, and many of the resorts have pick-up style taxis.
As another poster said - you can park your car at any of the (several) ferry terminals and get a speedboat (1200-1500 baht per boat, each way) or slow boat (50-60 baht per person each way) to the island. I prefer the slow boat as I never tire of the great views on the approach.
Of course, there are many different boats leaving Ban Phe for various beaches/resorts around the island.
Entry to the island (National Marine Park) is 40 baht for Thai and 200 baht for tourists. This fee will be collected by the Park Police as you step off the boat (or some of the ferries collect it as you sail across).
There are dozens of resorts on the island and prices range from about 800 baht to 6,000 baht per night per room. I believe there is also a camping site.
I've been visiting Samet about 2-3 times a year for the past 10 years and would recommend it, if you enjoy a quiet beach holiday (i.e.- not much nightlife).
My personal favourite is Vongdeuan Resort (on Vongdeuan beach) where prices are 1,500 to 3,500 per night (However, I've never tried any of the very expensive resorts).
If you want an island where you can take your car, try Koh Chang. The island is much larger and has proper roads (spectacular views if you drive from one end of the island to the other). But, be warned, they have a proper rainy season there whereas Samet seems to have much better weather.
Edited by chickenslegs, 2010-07-30 00:01:09.