Lidl in France is great. You never know how a major chain will do in a particular country. For example, Target's expansion into Canada was a huge disaster. But I would give any new chain a chance or two or three.
Generally, Lidl's selection varies, depending on what they're able to buy in huge quantities at a particular time. For example, we love San Marco coffee which is not stocked regularly at Lidl in France, but when it is, it's way, way cheaper than anywhere else.
They also have a decent organics line of products.
I'm sure their local flyer will be online when they go live so that you can compare.