As a Mac user, I often encounter have to open Windows programs that are not available for Mac. VMware fusion works perfectly, without occupying much of the resources. It launches fast and integrates well with the current system, allowing to copy and paste documents and text between the host and virtual machine. I personally tried every OS there is with VMware and have not encountered any compatibility issues.
I often run background tasks in my virtual machine and it would be nice to be able to lock it with password protection.
I recommend on not assigning to much RAM and CPU to your virtual machine if you are not going to perform any intensive tasks in it. This way your host and virtual machine will run smoothly
I often need to open files that business partners send me in applications that are only available for Windows (for example CorelDRAW, 3DsMax etc.). My Windows virtual machine launches in VMware Fusion within minutes without interrupting my workflow, so I don't need to reboot my Mac and boot into Windows. Thats saves a lot of time for me, so I can view a document within couple of minutes, quit VMware Fusion and get back to what I was doing before.
Also, when I develop a new website, I check the final product in different operating systems and in different browsers. And all of that is done within minutes. So VMware Fusion is definitely a timesaver.