Oracle delivered the first release candidate of VirtualBox 5.1 last week. The most significant revisions to the open source virtualization app are under the hood, and they add up to a better experience overall.
There hasn't been a major, overarching change to the VirtualBox UI, thankfully. The graphical client now works with Qt5, to make better use of whatever native OS features are available, such as full-screen mode on MacOS 10.7.
All of the UIs in the client, like the main machine manager interface or the Virtual Media Manager, have the same basic layout and behaviors, so there's nothing to relearn. But some Windows 10 behaviors don't yet seem to work properly. For example, if you try snapping a running VM window with the Windows key-arrow key combinations, the window either doesn't snap or behaves strangely.
Some of the other internal improvements speed up performance across the board. The APIC and I/O APIC subsystems, which govern system interrupts and allow the use of multiple virtual CPUs in VirtualBox, have been overhauled. Oracle claims this will yield "significantly improved performance in certain situations," such as networking.
But APIC bugs have shown up before. For starters, they once made 64-bit versions of Windows a lot harder to use in VirtualBox. That edition of the OS requires the APIC to run; now that the APIC has been fixed, using 64-bit Windows is far less problematic.
Windows users are winners with VirtualBox 5.1 in other areas as well. It now supports Hyper-V paravirtualized debugging of Windows guests, so guest instances of Windows can be debugged from the host using the WinDgb tools. This means those developing kernel-level drivers for Windows can test them out in VirtualBox and have access to the debugging tools normally only available when running Windows on a physical box.
Another significant change isn't so much reflective of VirtualBox itself as it is the world around it. Previous editions of VirtualBox had APIs to support automation of the app through C++, COM, Java, and Python. VirtualBox 5.1 has better support for Python 3, in line with the language's increasing third-party support.
VirtualBox is considered a free alternative to VMware Workstation, but because VirtualBox is open source, it's been used in other products that needed virtualization in some form. (Desktop tools for Docker have long relied on it.) The latest changes are good for both users and developers; they give the former a more stable and usable product, and the latter a greater advantage in building new apps.