I always give feedback for eBay purchases and have answered a couple of questions about things I've ordered from Amazon. I have reviewed a few QVC purchases, but after they rejected a 4* review that I submitted I thought "pfffttt" and haven't bothered since.
I do always read the reviews from other customers though if I'm buying something that I've not tried before.
QVC always bleat on about the fact that all reviews are checked by an independent group and that they publish them - good or bad. Well that's certainly not the case, especially as I didn't go against any of their rules. I'd ordered one of those small ottomans and a set of wheels. I simply mentioned that both could have been sent at the same time, rather than separately.