How well does VirtualBox perform when compared to VMWare Fusion for Linux guests?
Having extensively used VMware Fusion (all the versions from 1 beta till 3.1), Parallels (from their first public release till 5.x -haven’t yet upgraded to 6) and a lot of VirtualBox versions (Although I stopped using it six months ago), I can say that for Windows both Parallels and VMware ran circles around VirtualBox in terms of features, compatibility and speed. 

I used to compile .NET code inside those Windows and the speed of VMware and Parallels was superior (I went ahead and clocked different timings).

VirtualBox’s advantage was (and still is) price. For a lot of stuff VirtualBox is enough, but it’s still (as far as I understand) slower than its two (only) competitors. 

The advantage you have is that VirtualBox is free, so you can start trying it now, and both VMware and Parallels offer trial versions you can try. If you’re serious about finding the best virtualization solution for your scenario, I suggest you take the appropriate time to test all three options yourself. 

I’m personally using Parallels 5 these months, because it worked better than VMware 3.1 for my Visual Studio C# very big solution. However, I’ve used VMware and skipped Parallels 3 and 4 because they were more unstable and slow (for me). 

I didn’t want to upgrade to Parallels 6 because they annoy me with their $49 upgrade price that you have to pay every year.