I have just bought a GT JTD.
Firstly they look incredible in the flesh! I had seen a few on the roads previously and always thought them handsome cars, but up close the bodywork really is stunning. The bonnet seems to attract stone chips however if mine is anything to go by - you might want to check the ones you are looking at.
As above 16" wheels look terrible and spoil the look of the car. I went for a car with 18" wheels and feel they compliment the car perfectly. Despite their size I find the ride remarkably smooth.
Funnily enough I discovered yesterday that the hatch to release the front passenger seat to allow people to get in the back is indeed broken! Must be a common fault as mentioned above so better check that both front seats are okay.
If you are looking at cars on 07 / 57 then I would recommend the 170bhp variant - the 150 feels underpowered to me. In my mother's Astra sport hatch it feels quite powerful but mated to the big GT it lacks punch. I moved out to overtake some odd looking Proton on a 58 plate yesterday coming down a slip road who suddenly decided to put his foot down leaving me without the means to pass him. A Proton should not best an Alfa!
Other points to consider are that the GT is a long car with poor rear visibility so parking sensors might be handy.
Also whilst I haven't had enough drive time yet to optimise my economy, the manufacturers claimed mpg figures are miles out. I am currently averaging about 30mpg mixed though I hope to get this up a bit as I learn to drive the car.
Interior is almost schizophrenic in it's inconsistency of quality - comfortable and expensive feeling seats (leather is a must IMO!), lovely steering wheel, nice leather door cards and nicely detailed dials contrast with acres of cheap plastic, a stereo that appears to have been engineered by Fisher Price and a cheap and nasty feeling gear lever with an action more suited to a van. Plus because the mats don't anchor to the footwell, every time you enter or exit the car the mat slips up under the pedals.
As a bench mark I paid £9,500 for an 06 with 28,500 miles, leather, xenons, 18" wheels and winter pack with twelve months warranty, six months tax and twelve months MOT from an Alfa main dealer.
Hope you have a good day - will be interested to hear your thoughts after your day of test drives today.