Though they are initally popular with tourists, most people seem to stop using tuk-tuks fairly quickly. They have something of a reputation for taking passengers to an entirely different destination than the one they requested, and it's not undeserved either. This alternative destination is invariably a a type of shop, usually a tailors. They get paid a commission from the shop just for bringing you there (though whether they admit this to you is another matter), and this often takes the form of cash and/or a free petrol fill-up in a nearby station. If you find yourself in this situation, there's not much you can about it, except take a brief look in the shop, come out and wait to be taken to your proper destination. It's not a good idea to buy anything in the shop, the prices are marked up significantly (30% +) to give extra commission for the driver and profit for the shop, so they'll be cheaper elsewhere.
Remember too that tuk-tuks are no cheaper than taxis, and if you're offered any trip for what seems suspiciously cheap (e.g. 10B, 20B or even free), they have to make money somehow. You can be 100% sure this fare will be subsidised by either visiting a few clothes shops or as part of the infamous gem scam. The plus side of this is that you can often get to your destination quite cheaply, providing you're prepared to put up with a little sales pitch on the way. Some people even recommend telling the driver you know what he's up to before you get in, and you don't mind going to a couple of shops for him to pick up commission in exchange for a free ride ! Given that most tuk-tuk drivers English is extremely limited though (not extending much beyond 'Hey you ! Where you go ? You want tuk-tuk ?'), how well this plan will work if you can't speak Thai is debatable. On the other hand, it's not unusual for the driver simply to drive off after receiving his petrol voucher and leave you in the middle of nowhere - not a nice situation if you are a new arrival.
In general, the same rules apply to tuk-tuks as to taxis if you don't want any problems. Don't take ones that spend all the time parked in tourist areas, don't act like you've just arrived and problems are be unlikely. In addition, avoid offers from tuk-tuks for 'shopping trips' or 'sightseeing tour' for 20B/hour or similar, as it ends up wasting a lot of time. In the evening, when the shops are closed, is the best time to take a tuk-tuk if you want the experience.
