In reply to Father Noel Furlong:
The thing to remember with Decathlon is that they have a 'vertically integrated' product range. So there are low end things that are really cheap, but not so good, but their mid and top end things really are pretty good. None of it is rubbish, though. The higher the number, the 'better'.
As with buying anything, it's worth checking the cut and fit; last year, some of their 'higher end' outdoor fleeces had awful armscye designs, so that arm raise was very bad, and the fabric pulled across the chest. Shame, because the fabric was nice (R1 style).
Their Techwinter 500 shirts, on the other hand; if they'd flat-lock the raglan seams, they'd be passable as something from Arc'teryx; design, fabric, build. And if they added a simple grown-on hood, it would be a killer for the ME Eclipse/MEC T3 market.