My main gripe is that the cpu is too underpowered than I had realized. It really labors when using Handbrake. And, I seriously doubt it'd be ok if I were to run a guest instance under vmware.
I've got a Mac mini 2009 with the 2.0 GHz cpu, 4 GB ram and an OCZ Vertex 250GB ssd. It used to have the stock 120 GB hdd. The vm's are stored on an external firewire 400 500GB disk. Because the vm's are running on the external drive it does not really matter whether the internal drive is a ssd or a hdd. Either way, the mini runs various vm's at the same time really well. I can have Ubuntu, WinXP, Win7 and FreeBSD vm's open at the same time. However, I do notice when the vm's are doing I/O intensive stuff since that will slow them down but not my Mac mini. I also thought the 2.0 GHz cpu would be really slow but that it does it quite well actually. My old MacBook with the Core Duo 2 GHz and 2 GB of ram used to run vm's quite well as well but it was limited to 1 or 2 depending on what I was doing with other OS X applications.
My MacBook Pro early 2008 has nearly the same setup: 2.4 GHz Core 2 Duo, 4 GB of ram and the same ssd. It used to have a WD 320 GB hdd. On this machine I run the vm's from the same disk as I run OS X and all my other software (for obvious reasons

) and I can tell a biiiiig difference from using the hdd or the ssd. With a lot of I/O the machine would come to a crawl when using the hdd but it just keeps flying when using the ssd.
I'm guessing the I/O is what has the most impact by far on the performance of ones vm's. Running it on an external disk or on a ssd will give you quite a performance boost in both OS X and your vm's. I believe that I can get more performance out of the mini if it were to support 8 GB of memory, more than a faster cpu.
That same Mac mini is also driving a Dell 3008WFP using the mDP-DP cable from Circuit Assembly and does that really well. I'm really amazed at how well the Mac mini can keep up with the MacBook Pro. In my opinion both machines are really fast (and yes, 4 GB of ram and the ssd contribute a lot to that fact).