I'm not sure if the Shaker500 speakers are the same as the base model (minus the 8" door woofers). But they weren't super horrible. I was pretty set on doing a new stereo install though regardless. The mustang just could use a good 10'' or 12'' subwoofer for bass...I didn't like the bass from the 8'' shaker woofers.
The shaker systems aren't bad, but could easily be upgraded. I was talking about the base model, those 4 speakers cannot play at upper volumes without sounding terrible. I swapped mine out for some rockford fosgate 6x8's, along with a rockford amp that powers my 12" boston acoustic G5. (the same G5 has followed me from car to car to car!)