I miss quicktune as well. Going into all the different part menus each with different layouts, it's a pain, no more quick 20 second-ajustments... You could set up 3 different profiles as well - but it seems I need to buy an entire new car for that in GT5.
It's worse when you are doing an a-spec level before the tune as well, and forgot to buy, lets say some tires. You'd then need to travel all the way back to the part shop, or item page (why arn't my stored items available in the tune menu as well? :S)
Their UI isn't very well thought through, in my opinion.