VMware Fusion 2 was an excellent product. To a lesser extent, VMware Fusion 3 is as well. The current release suffers from some performance issues and as a result has created some disgruntled users over on the VMware forums. That said, a VMware employee has referred to the latest OS X 10.6.2 patch as potentially resolving some of the performance issues that Fusion 3 currently suffers from. With 10.6.2 seemingly close to release, I would hold-off buying any virtualization product until the verdict is in as to it's performance impacts.
I have to add that most of those disgruntled users caused the problems themselves by not reading the manual. They simply upgraded Fusion but not their vm's. Running out of date VMware Tools can cause performance issues to some extend and a lot of other different problems. This is not very bug with the smaller releases but with a big release like 3.0 there is a big impact as some features are changed/added/deleted. Problems seem to disappear like snow on a very sunny day when they update or reinstall the VMware Tools.
I'm guessing this is the case for about 90% of the users on that forum. You can clearly tell the difference between the inexperienced Windows users and the more experienced users that run different operating systems. The last part rarely reports problems and when they do they are more open and willingly to resolve the issues. It used to be a great forum but since 3.0 it has been taken over by noobs as it seems
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With 10.6.2 seemingly close to release, I would hold-off buying any virtualization product until the verdict is in as to it's performance impacts.
Luckily both Parallels and VMware offer 30 day trials and VirtualBox is free. However, it seems to be a good day to wait for 10.6.2 before buying (or not) one of those as it may resolve certain problems. Remember that Snow Leopard itself may be causing some trouble with some programs.
I tried Fusion and VirtualBox and do not like VirtualBox. It is slower and a lot more buggier than Fusion is. Just like Parallels, VirtualBox seems to be aiming at Windows support rather than being a great virtualisation product. I need the latter as I mostly run non-Windows systems like FreeBSD, OpenSolaris and Ubuntu. Ubuntu is not much of a problem but FreeBSD and VirtualBox is not stable at all. Each time you start the vm it's a big gamble if it will work or not. I've tried several new versions and the situation get a little bit better but it did not resolve the instability problem as a whole.
I also find the GUI of VirtualBox to be as horrorable as it gets. It's nearly unusable. You need to add the vm disk as well as installation disk to its own media manager before you can even use it. Trying to change the settings for a vm is also not very easy to do as it takes a bit too many steps as well as that it looks like rocket science. For most users that just want to run Windows in OS X this makes VirtualBox simply unusable and not an option. They have fine manuals, wiki, etc. which you most definitely are going to need.
If you want to relocate the vm folder you're in for a lot of headache. Both Fusion and Parallels setup 1 folder for each vm and put everything that is related to that vm in that folder. This means the logs, the disk and the settings file. In VirtualBox this is not the case. VirtualBox sets up different folders for all vm's: you have 1 folder for all the log files, 1 folder for all the vm disks, etc. Moving 1 vm makes this procedure turn into a headache. The usb support also seems to be flaky for some users turning that into a headache as well.
For what it's worth, Fusion has also the added value that it uses about 90% of the same code as every other VMware product does. If you already use a VMware product this can be very useful as you can exchange vm's quite easily without the need to convert the vm to some format. None of the other products have enterprise level products such as ESX(i). VirtualBox however does support the open format that is used in some products for the vm disks. In theory this should make it easy to exchange vm's.
Aero and the Coherence/Unity features are supported by Fusion and Parallels but Parallels seems to be doing the best in this area (it's a lot smoother and there are far less issues with it compared to Fusion 3). Due to limitations in the graphics driver VirtualBox is unable to do things like Aero (but most likely will be able in the future).
In the end they are all great products with some flaws. You need to try them in order to decide if the advantages exceed the disadvantages.
