1. The major reason why I'd use vagrant is to make sure that my development machine is almost 1-to-1 with the deployment server (hardware spec is basically too difficult to replicate in-office) this would solve issues between going from Apple's BSD to say a dependent machine which runs Ubuntu.
2. I can test vagrant scripts on remote clouds such as AWS or RackSpace with the same workflow which I followed on my local machine there is no rocket science to be learned if I am shifting from one hosting provider to other
3. As a person handling operations in my organisation Vagrant gives me a disposable environment and consistent workflow for developing and testing infrastructure management scripts. I can quickly test things like shell scripts, Chef cookbooks, Puppet modules, and more using local virtualization such as VirtualBox or VMware.
4. Personally I am working with Microsoft Azure azure provider made my life lot more easier compared to old style recycling of AWS EC2 Machines.
Sometime vagrant machines are very slow for no reasons mitigating them is the best option they are many blog posts on the same try to look into them.Apart from minor errors caused due to updates I like vagrant very much.
Users with windows client will see a tough time while using this product if you need to get most out of this switch for Unix/Linux
Best tool for people who need to work on different platforms without messing up with the existing system softwares,environments.Portable and Easy to dispose VM available on the vagrant cloud makes life easier of provisioning VM's and command line interface helps a lot working with multiple machines.