Eibach makes two different spring kits, ProKit and Sportline
The prokit is what I would consider if it was my car. 1.3-1.4" drop
The sportline has more of a slammed look and is 1.6-2.0" drop
You may find that lowering the car in general may make it a little more uncomfortable to drive daily and potholes/speed humps will not be friendly either. You certainly can drive a lowered mustang everyday though
I installed the Ford Racing K-springs on my '1o . They have a 1.5 inch drop. I was very happy with them. I drove it on crappy Long Island and NYC roads everyday and the ride was fine. In fact it didn't seem much harsher the the stock ride. You just have pay more attention when your pulling into driveways.
Plus One on what 84Ttop said. I've been daily driving on the same springs as Eibach Pro Kit for the past four years. (Had a set on my '06 and have a set on my '11) They are probably the ideal balance between looks, handling, and ride quality.