Yes, their slips are great. Also Heatgen thermals - much, much better than Uniqlo heattech - I know because I have some of those. The brushed extra warm heatgen ones (can't find them this year) are brilliant. Tights are good, I basically only wear their ultimate comfort socks, knickers are good.
I've never been a fan of M&S t shirts because the arm holes are too low - actually that's a fault with a lot of M&S things. Also the shoulders are often too wide. But that's because I have narrow shoulders.
I often like things from Autograph, and I reckon if they were styled differently they'd give a lot of the companies favoured on here (and a lot more expensive) a run for their money. A quick look found some cropped wide leg jeans that are exactly the same as some Me&Em had for £100 more, a striped jumper like Cos but half the price, another Cos-ish white shirt, and some trainers pretty similar but a third of the price of some that are always being featured on En Brogue.
As for the fabrics, I haven't noticed Cos (which I do, by the way, love) being the home of natural fibres - or indeed anywhere else that gets recommended on here. Polyester does have its uses - it's a wonderful fabric for trousers for instance. I have a beautiful dress made out of polyester crepe which drapes wonderfully and is both comfortable and immensely flattering. I don't get the M&S hate on here - when I look at the Hush catalogue I think, well, what a load of boring, boring dull clothes.