By gebobs in October, 2010
I do this trail on inlines several days a week. The Big Creek Greenway in Alpharetta and Big Creek park in Roswell are connected so the total round trip is a smidge under 15 miles.
http://maps.google.com/maps/ms?ie=UTF8&hl=en&msa=0&msid=115426453979635020450.00048d58b4a941af4e7b3&z=13
Pristine concrete, well shaded, one road crossing, a few moderate grades but typically level. The Big Creek Park portion has a few short boardwalks. They are relatively easy to skate on unless wet.