There isn't a Qdoba where I live. I travel to Sioux Falls often and always make time to stop at Qdoba. It's a place where I feel comfortable if I am by myself.
I always order the same thing - 3 cheese nachos with meat. I get them piled high with veggies and order a side of guacamole. The guac is tasty for a "fast food" type place.
Whenever I am here, it is always busy, but the line moves quickly and a place to sit is usually available. At Qdobas, you enter and place your order at a counter and it is made to your specifications as you move along the line. I have always been greeted with a friendly smile no matter how busy the employees may be. They have Coke products, which makes me happy.
They have a frequent customer card and give you specials on your birthday.
Qdobas is on a super, super busy road, a main road through Sioux Falls. It can be impossible to make a turn across traffic to get here from the west and forget trying to cross traffic when you are leaving to continue east. Plan ahead and arrive from a side street or from east.
The bathroom has always been clean.