I went there a couple of months back. I probably didn't give it a chance to be fair but it didn't reallycharm me. I don't ride a motorbike but I suspect if you do and youare able to rent one there (I guess you can as I did see a couple offarangs on one), you'll be able to explore more of the island awayfrom the town and maybe find a decent spot to relax. The hotels andguesthouses that are accessible from the town are either reallyexpensive or really terrible. I would have happily stayed for acouple of days but just couldn't find any accommodation within budgetthat I was prepared to stay in.
However ….. Si Racha which is themainland town you reach Ko Sichang from – I loved! So close toBangkok yet so Thai. It's on the coast but not the beach and theguesthouses are slightly ramshackle stilted buildings on piers righton the waters edge (similar to the old traditional ones in Hua Hin). You can get a double room for as little as 200 baht. The town has agreat night market, plenty of coffee shops and the park has freewifi! In the course of a week we saw less than a handful offoreigners; it's very traditionally Thai and not much English isspoken. Wonderful and so hard to believe Pattaya is only 30 minutesaway!
I'd say that it is a pretty interesting island, but mostly because it isn't a major destination. It is fairly well-developed, but not with resorts. Don't go for the beaches, though. If it had great beaches, it wouldn't be, as you say, off the radar.
Lots and lots of boat traffic in the area, though, so don't expect to be looking out to empty seas. It's in a major shipping lane.
I don't think you can compare it with Koh Samet. They are entirely different places offering entirely different things.
Sri Racha, as Kristy says, is a great place. Reminded me a bit of Mahachai, and Krabi 20 years ago.