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Had a decathlon 'so warm/light' bag for about 5 years now. Had around 80-100 nights use in that time, packs small, lofts up quickly, been washed a few times. Only niggle is length, as a medium sized man I thought a medium sized bag would be right, nope, 5'9 is really at the max of it, so their sizing is unisex just incase anyone makes same mistake.
Replaced a mountain hardware bag (stolen after only 2 years use) that was easily double the price (£300 vs £135 iirc), and apart from length (my mistake) and zip getting stuck a few times I honestly have not found a difference. Used them both the same places at same times of year.
Decathlon weighs about the same but down will compress much more than that synthetic https://www.decathlon.co.uk/down-sleeping-bag-trek-900-0-id_8575975.html