Nancy,
We use Solidworks for 75% of our sign structural designs. It is very strong program if you are going to be doing any level of custom structural sign design (Especially pylons). I would not recomend it for "Graphic Art" as there is better programs for this (Corel and Flexi). Also, we use AutoCad for Channel letters as Solidworks does not make it any faster/easier to complete those 2D designs.
The upfront costs is a high in comparision to AutoCad but the additional functionality is worth it, also your annual maintenance fees can be $1500+ per year per seat which can be tough to afford for some.
regards
Chris Sharpe