VirtualBox is a nice way to save 49$ for people in the know. VMWare Fusion only gives you the feeling you have something better because you had to pay for it.
People in the know will choose whatever virtualisation software works for their needs. VirtualBox and Parallels Desktop will have to import VMware vm's where as Fusion will simply start them. If you work with other VMware products this is easier. If you run more exotic operating systems it is Fusion you want because it has the best and most widespread OS support. VirtualBox is a good second since it is good in supporting Linux and the FreeBSD project is working with them to get FreeBSD to work properly as both the host and guest OS. Parallels scores points when running Windows because it is fast which helps with gaming. VirtualBox might be free but it can't do some things the other two can (try dragging files from and to the vm window) and has some serious issues with usb even if you install the usb extension pack whereas both Parallels and VMware work without problems.
The reason why I use VMware Fusion:
1. it runs FreeBSD, Linux, Windows, OpenBSD, Solaris without any problems; Parallels and VirtualBox have problems with these, the vm's crash quite a lot
2. it has no problems with usb and asks if I want to use the device in the vm or on my Mac; Parallels does the same, VirtualBox is a PITA with usb and not recommended when you want to use this (too often it does not recognise the usb device hooked to the vm)
3. easily exchange vm's between other VMware products such as ESXi, Player and Workstation; these are very commonly used products unlike Parallels and VirtualBox and importing vm's means converting them which takes time and is annoying
4. since 4.0 it is an app which means that all the services needed to run the vm's will only run when the app runs, not when you boot the machine like Parallels and VirtualBox
5. speed: it is faster than VirtualBox in all areas (GUI as well, I find that to be a bit sluggish in VirtualBox but it isn't that bad) and can match Parallels regarding Windows; if you use 32 bit vm's Fusion will use its own softwarebased vt-x which is very solid and fast, faster than VirtualBox and Parallels; when using 64 bit vt-x is a requirement in any virtualisation application btw
6. very clear GUI, better than Parallels (which can be a bit too much) and heaps better than VirtualBox. It is much easier to attach an iso or dmg file to the vm in Fusion than it is in VirtualBox. VirtualBox requires you to add it to some library, even if you are going to use it only once, after doing that you can attach it to the vm.
7. it has good (online) support: they have their own VMware forums and twitteraccount where they respond very quickly to questions. Also the support on other forums is quite good for VMware products. The fun part in this is that all of the VMware virtualisation products share the same base. Whatever goes for Workstation, Player and Server also goes for Fusion and vice versa. Parallels is quite sucky with the support, lots of people move to VirtualBox or Fusion because of that. VirtualBox has the standard Linux community support but it is owned by Oracle which means that there is no support from them.
8. everything is 64 bit
9. networking is solid: stable and fast although Parallels has more options for networking in this area. VirtualBox has good networking stuff as well but it can be a bit of PITA because there isn't much logic in the namescheming
10. I can encrypt vm's whenever that is necessary; none of the others can
11. Love the new snapshotting GUI in Fusion 4, makes much more sense and it is very clean (you can even branch)
12. select the boot device from the settings instead of mistreating the keyboard to get into the vm's bios so you can set the boot device, haven't seen that in VirtualBox, can't remember it seeing in Parallels
13. 3D graphics support, although Parallels is doing a better job for most applications (Fusion is better for a couple of apps, mostly in the CAD area)
14. Fusion has the best I/O performance, especially when you use an ssd (memory and I/O are the most important things for most virtualisation apps)
15. Fusion 4 now plays nicely with Time Machine which is quite difficult for virtualisation apps because of the size of the virtual disks
16. price tag: while VirtualBox has the best one (it is free), Fusion does a good job too. I'm using it since the very first beta version back in 2006. In total I have not paid more than 100 dollars for version 1 to the current 4.1. Parallels would have cost me for more than that.
The only things I liked about VirtualBox was its price and being able to virtualise 10.5 and 10.6. That last part is now gone since 4.1 and the pricing of Fusion was never that much of a problem, especially since there are many special deals, promos, etc. that can cut the price in half.
All in all I think Fusion should be very aware of VirtualBox. If it improves the usb support and makes the GUI much more simpler it will be a very very competitive product! Parallels Desktop is a nice product if you run Windows since it brings lots of features and speed that the other two don't have. I really like that things are very close, keeps them sharp and innovation going.
They'll never approve publicly but will never do anything to stop it.
Apparently nobody in this thread has even taken the time to read the originating article at Macworld. An Apple representative confirms that the license for both 10.5 and 10.6 allows for virtualisation. The 10.7 license is just more specific about it. It is the last paragraph:
An Apple representative told us that Apple's end-user license agreements "permit properly licensed copies of Mac OS X Lion, Snow Leopard Server and Leopard Server to be virtualized on Apple-branded hardware only." It's unclear what Apple's disposition toward VMware will be. In the meantime, this update gives Lion users an outlet for running PowerPC-based apps, and lets developers and other technical users have easy access to multiple OS X versions on a single Mac system.
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...with Fusion 4.1 yesterday. All installs are smooth. Only thing I noticed was there is no audio in 10.5, but its no biggie
pac
By default there is no audio in 10.5 and 10.6 but there is a way you can get it to work in 10.6 (no idea if it works in 10.5 as well, since these two are nearly identical I'd suspect it will). In the VMware forums there is more info about the matter:
Fusion 4.1 and Mac OS 10.6 sound and resolution issues. Tried what it says there and it works in my case.