I understand that the most widely used is AutoCAD from Autodesk. Other software are also compatible with AutoCAD .dwg formats. CAD software that has the same interface that is like ProgeCAD and GStarCAD. The biggest difference is the price that was taken in addition to various features.
Above is Autocad 2008 classic interface screenshot. More recent versions have been using Microsoft Office 2007-like Ribbons. I think Autocad performance with this interface is quite good, does not feel slow like the 2009 version that has been using ribbons.
What is the differences in interface the two pictures above?
The above image is a ProgeCAD 2010 Professional interface screenshot. Almost no difference with AutoCAD interface. Even use are the same. So also with GStarCAD (screenshot below) and possibly other similar software. ProgeCAD have a product that is free of charge for personal use on smart series. The latter is ProgeCAD 2009 Smart but not support Windows 7 (Seven).
As seen, three of this software has an interface similar and perhaps others as well but the price is so different. Users do not need to adjust because its use is also the same if switch to using CAD software other than AutoCAD.