I will cut the suspense right away: not working very well.
Actually all my Linux VMs on the Mac have not worked well with VirtualBox. I have tried many version of VirtualBox and there is not one that worked better for me.
I searched on the community forums on the virtualbox.org site but did not find much help.
The experience of installing RHEL 6.1 in VirtualBox was not easy at first because I was allocating the maximum memory it would allow me. I have plenty of available RAM on my iMac so that was not the issue. I was monitoring with the Activity monitor app and we were not hitting any limits from what I could see. In the middle of the installation VirtualBox would give me an error about the host being low on memory and would pause the installing VM. Only when I dropped the memory allocation from 3500 Mb to 2048 Mb did the installation work to the end.
The VirtualBox tools installed no problem but since I did not use the VM enough before installing the tools I am not sure if it was stable in the first place. If I start Firefox and Eclipse it freezes and I have to force a shutdown of the VM.
VirtualBox works great on my Windows 7 laptop and I have no problem running many apps in it. It is actually a very good development environment for what I do.
On the Mac I am afraid that I will have to find another solution and that is too bad because the iMac is a much better machine than the laptop to test RHEL 6.1.