As outlined above, there are plenty of forums discussing the relative merits or petrol or diesel. I eventually chose the diesel route and with accompanying Angel remap and have to say that I've not been disappointed... It obviously doesn't sound particularly tuneful (although it's far better from the inside than outside) but that's a small price to pay for the mid-range punch the engine gives.
The car is a decent size inside although the doors make access in tight car parks a bit of a pain and as mentioned above, getting kids in and out can need a patient approach. The boot is easily big enough to carry sport equipment - I carry golf clubs and a trolley regularly and there's plenty of space for that (and extra).
Think the things to watch out for have been outlined above - buy one from a indie if you can. Make sure that the waterpump has been changed in line with guidelines and when test driving, check for strange knocks and bumps from the rear (the boot / exhaust / rear bushes can all be noisy).
However, have to agree with Paj - the GT is a great car and if you buy one, you'll have a smile on your face every time you go to drive it ;-)