Well, I got lots of advice on here before I went, so the least I can do is give you my own perceptions...
Arrived on Samui 3rd Jan after 3 nights in crazy BKK. Weather was cloudy, apparently some bad weather from China had just passed through. 2 weeks of unbroken sunshine after that though, and the sea soon calmed down & was very calm & clear. Booked into our hotel - the Imperial Boathouse. Rooms were a little dated but had everything we needed, the gardens & pool were absolutely beautiful and the staff very obliging & helpful.The balcony was almost as big as the room. Although we heard some occassional noise from the building site next door, it certainly wasn't enough to spoil or day, let alone our holiday.
The hotel grounds lead onto a fairly quiet and absolutely lovely beach, a few vendors selling the usual crap who soon learned to pass you by when they knew you weren't going to be buying, and a few restaurants who set up tables on the beach for evening meals. Dishes started at about 1 euro, and eating dinner with the sand between your toes, a cold beer, no rave music.......life's good.
Choeng Mon was fairly quiet, which was just what we wanted. We went into Chaweng after a couple of days, and I have to say if we'd booked a hotel there, we'd have been moving out the next day. Chaweng wasn't for us at all, a busy and desperately unattractive little town, a crowded beach - I really don't see the attraction, although there were loads of younger people there who looked happy enough, so it must have something for somebody. Being the biggest resort, it gets the most guests so I can see how so many negative reviews of Samui could emanate from here. But believe me, away from the tawdry towns, Samui still has some lovely spots of unspoilt beauty. Choeng Mon bay, and a lot of the south of the island are absolutely gorgeous and much more suitable for the discerning traveller. (and me!).
We did just a few trips, the Island Tours all-day ticket was decent value for money, a wee bit of elephant trekking, snorkelling off an island at the south, up to viewpoints near the Secret Garden. The trip to Ang Thong is an absolute must too, stunningly beautiful and I was never happier than when kayaking round one of the little islands, looking at tropical fish, caves, just bliss. I'd strongly recommend staying overnight here - there are a very few simple bunglaows on the main island - and a few tents for hire if they're full- and the peace & quiet when all the day-trip boats leave at 3pm is just heaven.
Shopping for the usual souveniers - fake rolex, copy designer gear etc was pretty much on a par with BKK, price wise & to a point choice-wise too. There are a LOT of tailor shops around - too many for the amount of custom so they try very hard to get you in. Stay away from the too good to be true offers & you can pick up some nice clothes, well-made in good materials, at a fraction of the price back home.
I had some doubts about our wisdom in choosing Samui because of some negative reviews here. Having now seen Chaweng, and indeed the Bophut area too, I can see why some people would be really disappointed in Samui. But it's totally unfair to paint the entire island with one brush. It's still possible to find little bits of "real" Thailand here, although I wouldn't leave it another 5 years to go, as there IS a fair amount of building going on.
The weather's great, all the hotels I saw were of decent standard, the people are exceptionally friendly - even the ones who don't try to sell you anything!, some fab beaches, cheap but very decent food.
Choose your hotel carefully - Chaweng would be too busy for some, Choeng Mon too quiet for others - but you can still have a great holiday on Samui.
Feel free to PM any questions on specifics, I'll do my best to help.