Big Budda is worth a look, I had heard it was in a bad state but it has obviously been painted and looks quite impressive- especially from across the bay.
I found Chaweng to be too busy with a poor beach, though if you're in to lively nightlife its the only place to go.(Worth a trip to see Thai Boxing).
We stayed firstly near Bophut and then at Hat Choeng Mon. Bophut is a really nice village with great eating places, and cheap accommodation. Also many boat trips leave from here so its very handy. The beach isn't the best, though we were never without a moped and found some great places not that far away.
Choeng Mon is in a different class, Great beach still with little fishing boats- not at all spoiled. Great food at night - all the restaurants serve by candle light on the beach- truly beautiful. If you aren't that into exploring, its the type of place you could stay for a blissful few days and not need any thing else. (We stayed at Choeng Mon Bungalows and had an air conditioned chalet with hot water and Sky Tv for about £8 per night).
Quick Tips:
A candle lit dinner on the beach at Choeng Mon is not to be missed.
Best Way To Get Around:
The roads are quite good on Ko Samui. You can drive right around the coast. A moped is a cheap and easy way to get around (can be hired from around £3 per day)