Generally Decathalon is a great shop, although it can be a bit hit and miss, depending on what you're looking for.
It's superb for cheap fleecy items - you can get a fleecy hat or pair of gloves for literally £1 or a couple of quid. It's also good for general sports tops and the like. Basically the sort of stuff that you don't need to be too technical, and don't really care if it gets wrecked. Although credit where it's due, the sports tops are certainly hard wearing - I've had mine for years and they're still holding up well. I've also bought sports bras and running socks here, and they are decent enough.
What I find it's not so good for is for the more technical items. I wouldn't buy a base layer from Decathlon. Or items for curvier ladies. I think they might be a Spanish company perhaps - which generally means that their own brand version of 'large' is certainly not a UK version of 'large'. My recent attempt to buy skiing salopettes here did not end well. I got them on, but I sure as hell wasn't going to be able to ski in them! And I'm only a size 12 - clearly gigantic by continental standards.
They also seem to stock a wide selection of kit although again that seems a bit hit and miss - they didn't have any sparring gloves that were suitable for my purposes, but Simon H bought a great pair of skiing boots for a very reasonable price, and we did also have our eye on a mighty fine inflatable kayak at one point, which we haven't bought. Yet. read more