Received mine yesterday and wore them on my commute this morning. First impressions are good - they fit well, are stretchy enough, the poppers to tighten the ankles work well, the reflective details are subtle enough. Can't comment on the showerproof-ness as it was dry.
Two big plus points: they only cost £39.50 and they come in larger sizes. I have no idea how they compare to similar offerings from Rapha or Vulpine, for example, but as I can neither afford nor fit into them it's not a comparison I can make. Maybe one of the cycling magazines or website will review them but maybe M&S haven't sent any out - until yesterday they weren't even being sold as cycling specific even though there's a big label inside that says "Cycling Chinos".