I had a GT and I loved it. It sounded fantastic. I have had a few exhausts since and none of them sound nearly as good. While they are built in Korea (IIRC), the build quality was very good.
The connecting pipes are worth it if you plan to go back to stock at some point (no need to cut your stock one). You could pick up a used one for $150 and hack that one up though.
The reason that I sold mine was that I wanted to run an x-pipe with HFC for the power gains. After trying a few different x-pipes, the GT was just too loud for me (and I don't mind some good volume). It sounded like a race car though!
2008 IB E92 M3| BBS | KW | Arkym | Platte Forme A.G. | Active Autowerke | K&N | Fabspeed | Dinan | Evolve-R