I've just bought an Alfa GT Blackline (6 weeks ago) and so far I've not come close to regretting it. Mines the 1.9JTDm and it pulls strong and is surprising quick at times. Fantastic leather interior with red stitching and brilliant Bose sound system. I don't think you can beat the diesel for motorways and overtaking.
I've never owned an Alfa before and everyone says they're not the best put together cars, now I've always bought VW's and I don't think the build quality on the GT is far behind.
My advice is make sure you get a good run on the test drive.
Check for vibration (steering wheel) and noise at motorway speeds (and above
Check there's no pulling under braking.
Check the clutch on take up for judder.
Check the gearchange is smooth and not notchy.
Check ALL the electrics no matter how insignificant the function.
Check the car doesn't pull to one side when you let go of the wheel.
All stuff you'd check on any car really. I always try and get good light as well to look for car park scrapes and dints.
I thought if it is crap and breaks down, at least it'll look good at the side of the road and the interior while you're waiting is gorgeous