i would not call the dsg or the smg a slush box. i also would not consider them more fuel efficient or semi automatic.
there are a lot of other semi automatic transmissions out there that are complete garbage. only audi/vw(dsg) and bmw(smg) make the good ones. for anyone that is a real driving enthusiast, please test drive a car with dsg or smg before putting it into the same category as the other manumatics. you will be happy you did.
cvt is another issue.... i am not into it, but, in theory, it definitely is a good setup for fuel economy.