As a mustang owner, I shall try to give you my most unbiased opinion.
I took delivery of a brand new '06 Mustang GT, got it with .5 miles on the odo so no other previous owners fudgin her up.
I took her home around the end of February, and now she has about 11,500 on the odo. I drive mostly to school/work/church and that's a mixture of highway and local. And a road trip to Atlanta. I drove there straight, all 14 hours, only stopping for gas and food. Nothing was out of line and everything was just perfect. Got around 24mpg cruising at around 80-85mph.
The stang I can honestly say is alot different from the other cars on your list. The stang is more muscle car, while the others are comfortable cruisers. It all depends on I think what you want. *Side note: If you're looking for the fastest out of them, the stang wins I have raced all the above expect for the A3 on multiple occasions and won w/o any problem.
So far I have not had any problems with her so far. She's a pleasure to drive anytime and honestly, you get alot of compliments/looks when you're cruising around. Now, no car is perfect and there are a few small minor problems.
1. Under hard braking, she kinda leans to the left (or is it right? I forget at the moment). At first I thought i was just on a bad groove in the road, but I've talked to other owners and they have it as well. It's not horrible, but its there fyi.
2. The gas tank is horrid. At some pumps, the gas will come out too quick and if you're not careful, it will spill out everywhere, even if the tank's not full. Apparently, there was a recall for this and it should have been fixed for the late '06 and the '07 models. Mine is perfectly fine now too.
3. This seems to be an isolated experience, but at one point, all the electronics went haywire. The doors would lock/unlock themselves at random points, all my warning lights came on and off and so forth. It lasted for about a day like this. The next day on my way to the dealer, it stopped and never came back. Oh well.
4. This really isn't a problem, but alot of people complain about the throttle lag caused by the drive by wire system. You don't really notice it too much, unless you punch it from a roll. There is a custom tune for it that will fix it of course.
Other problems I have yet to encounter or just forgot at the moment but honestly, it's still a brand new car and this being my first ford, I can't comment on its long term reliability. I mean so far so good though, and I'm sure that Ford won't take any chances with the only car that they seem to be selling at the moment.
Anyways, I'm at work so that's all I can come up with at the moment. If you have any questions whatsoever, feel free to PM me.