Can't say that we went to 'Castorama', never actually came across one to be honest. We found Leroy Merlin to be about the best and some Mr. Bricolage offered decent enough quality. It completely depends on what your looking for in terms of spec etc.
TBH the Brico Depot offered less in terms of quality but their prices are rock bottom, again depending on what you are looking for, but we didn't make any purchases here.
Definately carry out as much research as you can on the products. We had been to Brittany a few months before on a holiday/research mission, so we had a fair idea of where we were going. If you travel to different towns, don't discount a store because you were in one in another town as we found that they carried different things in each branch ie a wider selections of showers or stoves or whatever....
We concentrated on St. Brieuc (Brittany) and surrounding areas. Do learn a bit of French if you don't have any. I speak a small bit from my school days and it really did come in handy. We found my bad french broke the ice and shop assistants were apprecitative and very helpful...(probably out of pity

)!!!
Plumber has had no problems, just had to let him know b4hand that we were intending on going to France for bathroom ware.........
Let me know how you get on...if I can be of any further help, just let me know....!
Bobbins.
(absolutely no connection with any of the above mentioned companies!)