GHD's are definitely worth every penny. Every owner of a pair of these ridiculously popular straighteners will tell you they changed their (hairstyle) life! No kidding. If you haven't had a chance to sample a pair through friends, family etc please take my word for it now.
I think its mainly down to the fact that they are a well made and work so quickly. Because the ceramic plates go up to about 200 degrees C, they literally straighten on the first iron (without having to keep going over and over the hair, like with most other crap straighteners!). The great thing about GHD's too is that they make your hair not just straight-ish, but poker straight. You won't feel cheated with them by any means.
If you have really unruly hair - ie not regular curls (which actually look nice) but sort of kinked hair the GHD's will make such a difference. You will also find that you probably won't need to wash and style your hair as much - the straightened effect lasts maybe a few days and stays really shiny.
Although GHD's cost from about £65.00 for the mens mini styler, and £70.00 upwards for the original straightener (the most popular model), they really are a great investment. Many websites are now offering a discount price on GHD's so you don't have to get them from a posh salon (which may charge £90.00 or so for them).
A good tip with your newly purchased GHD's is to make sure you always look after them. Don't wrap the cord around the appliance EVER, because this will twist the connections and affect the heat plates. They might even break for good if you carry on doing this! I'd keep them in their box (which is like a very sleek box indeed).
Go on, buy a pair now!