Their return process can be a hassle in the rare event that something arrives damaged, but I totally agree that they're awesome. Just about everything arrives pristine and (knock on wood) I've never received a spine slash from Fnac.
I've (begrudingly) got the Superman set preordered with Amazon, too. Fnac's release date is listed for April 26, so it you preorder from Fnac then you'll have probably it by the 27th. If it's mint then you can just cancel the Amazon order. They won't have charged you yet anyway since the US release was pushed back into May.