Having just mentioned ESXi it only seemed apt that Ubuntu should maybe also get a mention too in keeping with the whole Linux thing. Generally Ubuntu has come a long way in the last 3 years in terms of its evolution as a general user experience. In its current 11.10 form (that I'm currently running in a VM) it seems to have very much so moved toward the kind of thing that most people would more readily identify with Windows or OSX whilst keeping it all relatively fast and light. Many of the command line driven interactions have pretty much been replaced for simpler point and click command interactions although the option to use command line interfacing is still there for all the more die hard Linux users.

Visually I think most people would find the latest Ubuntu GUI environment very pleasing to the eye and obviously if you're willing to go to the extra lengths of getting to grips with some of the more advance features of Linux coding you could customize it any way you like beyond the options that come included with Ubuntu. The best part? Well the Ubuntu team have pretty much already figured out how to make much of your hardware work within the Linux based environment as a host OS, that also includes getting the specialized features of your Nvidia or AMD ATi graphics card working in Ubuntu too for enhanced hardware accelerated GPU rendering.  

Secondly since VMware  applications are built on Linux whether that be via Workstation for the PC or Fusion for the MAC (both exactly the same thing but with different names) a Linux host environment would potentially provide an even faster and smoother operating environment to run VMware in. Not to mention the fact that there's a specialised Linux based install of VMware too.

Generally though Ubuntu does seem like a relatively safe option for general users wanting to test the Linux waters given that it has support from many large well known and reputable IT industry names which you can get a run down of on the official Ubuntu website. There are some decent Linux based apps and Ubuntu have also been prudent enough to build an app store like application that significantly helps simplify the download and installation process even further which includes auto unpack and auto install. However you will generally find that it will be nothing of the variety that you'd be more accustomed to when using Windows, but since we might be setting this up as a GUI based host shell for virtual machines it isn't really going to be too much of a problem if you've got a Windows based VM and the appropriate hardware resources on your machine to run it all on.

This thing was extremely fast and light running from a VM with only 1GB of RAM assigned to it and a single core of a dualcore processor whilst using the same hard drive as my host windows OS. I can only imagine how much faster it will would be as a host OS. 

Security considerations (it wouldn't really make much of a difference)

I'm generally a little dubious of anything that’s open source since despite all the claims of Linux being a very secure platform due to the fact that much of the malware and viruses designed for screwing Windows up is simply just not compatible with Linux, there's also the fact that there are many-Many Linux coders that know Linux inside out, upside down and back to front!! Meaning that a Linux system could still potentially be compromised even without the need to even use malware or viruses in the purest sense. Meaning the only way that you'd be able to have a truly secure Linux system is to be an expert coder in Linux and be well versed in the workings of it yourself. In fact if you were an expert coder in Linux you could also potentially make Windows and OSX even more secure too. I might also point out that many of the hardcore hacking experts do in fact use Linux for hacking/cracking Windows and Unix based environments, the difference of platform is barely a hindrance to proficient Linux coders. Click here to go there now. For all those that weren't aware, 12.04 is still in beta phase but its perfectly possible to download the stable 11.10 build and upgrade it to 12.04 final via the Ubuntu update procedure when it finally drops for general acquirement. Also tt shouldn't really need to be said but if you're a gamer you might want to stick to Windows as your host OS for obvious reasons of end user compatibility ease, unless that is you intend to run Ubuntu as an alternate multi-boot option for dedicated VM use.