I usually only take my dogs with me if they are going in somewhere (like, going to my mom's house or to the pet store, occasionally when I go through the drive-up window at the bank because they have dog biscuits).
However, if the weather is nice and it is not too sunny I have no problem leaving them in the car for up to 30 minutes. I would never leave them unattended in a hot car, or even on a day that was over 50 degrees F. But, they like being in the car because they like to watch people going by and getting to see new things. Then, when I'm back in the car they like to check out what I bought. I just make sure to lock the doors, take off their seat-belts and leashes when I am gone so they don't get tangled and of course have the windows part-way down!