Their kit is solid if unexciting. Not quite the VFM it once was imho but certainly still good value.
An issue I have with it (and I risk just accusations of snobbery here) is that near on everybody here in France wears it! You can't go anywhere without seeing Quechua fleeces or shoes etc. But, on the other hand, that speaks volumes; it does a job at an affordable price.
Personally, I don't like the Btwin range of cycling gear. There's something very uninspiring about it and I'm not convinced (looking at it) that it'd suit my purposes. I use Decathlon for consumables on the bike (tubes, chain oil, etc), and garden/walking dog clothes but little else.
As another frenchie (or economic migrant would be the more modern term) the one comment about D4 clothing and sports shoes is that, yes they are worn by just about everyone in france but
but they are mainly worn as cheap everyday clothes, not for their officially quoted purpose as sports clothes (although school sports do give them a big market, which is more a measure of how bad french school sports teaching is). OUAT they had a wide range of useful products but they have tended in recent years to go towards mediocrity.
They don't go out of their way (at least in Poupou country) to sell bikes to which you would want to fix panniers for going out of town for pleasure for several days. In light of this I would be wary of the quality of their panniers; it is not a field in which they have form. I would not expect their local sales staff to know the first thing about using panniers or fixing them to the bike.
I have some bargain basement panniers from SJS, both large and small, that were pretty cheap and have proved their worth both with me and (more importantly) with others to whom I have lent them. The big ones don't seem to be available any longer but the small ones do come up from time to time.
I have a D4 sleeping bag that I have had for some years (about 18 or soIIRC). It is not as warm now as it once was!