Ive had no problems with my fluvals but I have read a good few posts on here with people slagging them off. Most of my tanks are Juwel tanks so they have their own built in filter, the only two other filters I have in other tanks is the fluval and an interpet PF2 in my lobster tank which Ive found to be fine.
Problem is that the shop might not replace it since they might not be able to test your one against another one in the shop without submersing them in water, the SHOULD replace it but it depends where you got it, if its a resonably big shop or a chain you probably wont have a problem, if its just a little shop with some 14 year old behind the counter who couldnt tell a guppy from a goldfish they might try to fob you off.