My first experience was with GNS3 and I remembered when I made my Tutorials and wanted to share it with Cisco Academy the Manager told me GNS3 is not standard! and now it’s famous and can use it even from inside the Cisco Academy. After that Cisco recognized how simulators is very important! but GNS3 still more wide than CISCO VIRL. All IT guys move towards open source software and if CISCO VIRL become open source will kill GNS3..I think!