Aftermarket cats are bad news. I have seen so many faulty that I no longer fit them. The problem is that a lot of the well known exhaust brands are outsourcing production.
A diagnostic specialist friend of mine recently had trouble with aftermarket cats on a Scenic and eventually fixed it by fitting a Walker brand one. Dont take this as a reccomendation, just in case
As hi5 said above, cats usually go because of a fault in the engine management system (lambda sensor or otherwise) so it is vital that this is looked into or you will be in the same position again soon after. The reps from the exhaust companies will always try and say that the car was the cause too, not easy to prove unless you or your mechanic/tech is able to hold his own in a tech type conversation.
P.S. I see quite a few wrongly diagnosed so be sure that it needs to be replaced first.