I did Gap Medics pre-dental in Thailand last winter break (2 weeks). I observed in a dental clinic inside a hospital and got to see endo, oral surgery, lots of stuff. Also, because you're paying to be there, the dentists really tried to help you and I felt like it was way less awkward than shadowing stateside sometimes could be. The program is definitely mainly geared toward pre-med but I wouldn't say that's a bad thing. Everyone there was cool and since there were only two other people doing dental who showed up after me, I got plenty of quality time to talk to the dentists. I expressed an interest in public health and one of the dentists there even introduced me to some people she knew who worked for the Thai government doing public health dentistry and I ended up spending a day with them and later doing a little public health research project related to that the next semester when I got home. The experience plus the project played really well in interviews.
So overall it was an awesome experience and I highly recommend shadowing overseas..... BUT the program itself is - as you might already suspect - kind of a scam. They overcharge you like crazy considering the cost of living in places like Thailand. For what I paid to be in some crap dormitory an hour from the clinic eating cheap food, I could practically have been living in the 5 star hotel right next to the hospital ordering up room service. I kind of knew I was getting ripped off going in, but I forked over the cash anyway because - as you said - there aren't really any other programs like it and it's very convenient since they set everything up for you, take you on little outings and all that. It's also fun to hang out with all the other pre-med/pre-dental people.
If I had to do it again... Depends on how rich you're feeilng and how much you care about the social aspect. It might sound a little scary, but if I had to do it over I think I would just cut out the middle man and do it myself. In other words, fly there, get a nice hotel, and then go to a big dental clinic and offer them some money to let you shadow for a week or two. It might be intimidating but you would probably pay WAY less than what gap medics charges you.