The ease of use, how simple it is to setup VMs for testing or development purposes with very little configuration required! I was able to get up an running in minutes without any previous experience using this product as there are many tutorials available online that walk you through each step along the way.
It's not perfect but we've had no problems so far (and have been working together since 2010!). We primarily work within our own environment at my workplace however sometimes when one member needs access from home they will need their VM set-up remotely by another team mate who doesn't know what he/she has done previously - which can be frustrating if something goes wrong!! This helps resolve all those issues whilst still providing them remote support where needed!