I user VirtualBox all day every day for fairly complex tasks, and it has performed admirably, yet it is sorely in need of QA help. Major releases happen with auto-update notifications and then you realize that your old snapshots can't be started, using a debugger blows up the VM, sometimes snapshots don't save properly even though it looks like they did, etc. etc. Then you have to dig out the last working version, which came out 6 months back, to get up and running again.
Aside from this "upgrade gamble", which I put squarely on a lack of beta releases, VirtualBox is fantastic. Hardware accelerated graphics with full Aero support, fast virtualization, shared clipboard and files, attaching USB devices - it's everything you need in a friendly UI that anyone can work with.
It'll be a tragedy IMO if it's left to rot.
For anyone interested, I find the last stable version to be 4.3.12 (on Windows).