Personally I think you get what you pay for with straighteners. I've tried so many different types and nothing compares to my GHD's. The cheaper ones definitely don't seem to be able to straighten the hair as well as the GHD's or even keep it straight all day, like the GHD's do. Also the new GHD's with the rounded barrel you can curl your hair, (if you want to), the earlier versions it was harder to do so as it was more square. I also like the fact that I can take mine on holiday to the US as they've got a dual voltage, whereas in the past I had to buy a cheap crappy pair for holiday use and my hair always looked awful.
As already mentioned, the hotter they get the easier to straighten your hair, plus if you use a protective spray beforehand and get your hair regularly trimmed there's no reason why GHD's should damage your hair. If you do decide to go for the GHD's, shop around or wait til after Christmas as that's when the offers are about.