Yes to dirty hair. Holds much better. Do not wash hair the day you want it curled. The day before wash your hair is ideal.

And second the cloud nine straightener. Though ghd just released some colored ones, they are lemon, aqua and purple colour and these ones have the original technology in the plates like in the beginning. The ghd 'gold' versions (the most recent) weren't as good. BUT ghd are about to release a new one... Supposed to be the rolls Royce of straighteners.
(Sorry for the hijack there... I've been looking at buying a new one myself.)