I'm using the extremes and find them great. I'd agree that they feel like they're not doing much, i think thats the idea of them, really easy to use. I'd also agree that for heavy soiling, 1st cleans, heavy scrubbing on UPVC they probably aint your first choice.
I've got what i think is one of the early extremes, its a monofilament brush, quite tough and has great scrubbing power, and still the low weight of the extremes. I am looking to get another one, Gardiners don't seem to sell them anymore. Anyone seen one or got one ?