The reasons I use VMWare Fusion are:

1. VirtualBox shared folders performance is terrible.

2. VMWare Fusion is the only shared folders implementation I've found (tried Parallels, VirtualBox, and VMWare Fusion) that can follow symbolic links, making them look like normal files/directories on the guest (useful for Windows guests which don't understand nix symlinks).

3. IIS can't host applications off of VirtualBox shared folders for some permissions reason, which cannot be fixed.

I really hope VMWare Fusion doesn't die, as it's the only product I've found that supports my use case.

Can't stress enough point #1. I've seen a HUGE performance difference between VB and Fusion with actual web-development workload. Enough to justify moving from a free solution to a paid one. Plus you get other nice features, eg. open a file on Mac OSX with a program installed in a Windows VM.

When they discontinued VMware Server 2, I migrated over a dozen machines over to VirtualBox on CentOS. We have had no issues with performance or stability with any of our machines. VirtualBox 5.0 brought feature parity to VMware Server for our needs. The only thing we have found lacking are more remote management features.