ive got one but never really used it much due to fact im not sure the best way to do it.
doing it right is a lot more complicated than you first think,do you sit all night in it..do you do high reps..how far do you stretch ...how often...do you force against it.
ive always stuggled with flexibility and the fact is its a very complex subject,by doing what you think is right you cuold be doing more harm etc. ive got a book on scientific stretching and its mindblowing.
you need to know what stretches to do for a specific outcome ie high kicks,static flexibility,speed etc.
So really the quick fire answer would be yes they are good if used correctly to achieve a specific goal they are created for.
But how to do it aint got a clue :-)