I could probably dig up more of the same - other people's views on the flowmaster catback. You're either going to listen to them or not, so w/e. Don't get me wrong, flowmaster makes other good products, but it seems like their catback would be worth shying away from when there are others out there that are affordable and better performing.
Now, all I have info on is relating to what would go onto a 4th gen f-body since what I drive is a 99 z28. Results may be different for other cars.
But really though - if you have an X-pipe going into a catback, you just turned it back into a Y-pipe. I mean, that's what catback commonly refers to. If you're talking about adding flowmaster mufflers to do a true dual setup, that's a completely different story. If that's the case then flowmaster might be just fine. I haven't looked into their plain mufflers. But you will confuse whatever shop if that's what you're looking for and you call it a cat-back. True dual will have the best performance, so I'd highly suggest doing that if you thinking the cat-back was what you wanted.
As far as alternate catback, based on other people's reviews and cost, I'd suggest a Hooker cat-back.
And since I don't see it mentioned, what sort of car is this for?