Its has a many novel and very productive features compared with AutoCAD including some that are especially useful for drawing isometrics, P&IDs etc. Its not free but IMO its the best tool for the job and one of the best 2D drafting programmes for both engineering drawing and non-scale 'sketching' such as isometrics and P&IDs. The best CAD software for 'manual' (rather than automated) drawing of isometrics is Smartsketch.
