GHD and CHI are about the same for me, except my CHI got stepped on *cough* and I don't feel like testing my GHD to see if it withstands a boyfriend's foot for comparison >.>
The differences between the cheap and expensive ones are:
ceramic plates, and higher more durable quality ceramic (cheaper ones sometimes only have a paint coating that wears out and can damage hair)
fast heat up (I remember my curling iron back in junior high would take 5 minutes before it was hot enough to make a curl and then would cool off by the time I was done with 2 or 3 curls, so it'd take over half an hour to curl my hair...my GHD heats up quick and stays hot until it auto-shuts down or I turn it off.
What flaws do you know about with the GHD and SEDU (that the cheaper ones don't also have)? just curious. The only thing I can think of is that the more expensive irons will get hotter which can cause more serious burns to the skin as well as easier heat damage to the hair, but at that point it's more about user technique than machine.