If you're willing to spend time learning how to use Linux, which actually entails reasonably little in quite a few distributions (ie Ubuntu), then I'd say Linux for money saved.
However many just want a setup that can be said to work out of the box (within reason) and require very little learning, I'd say Windows 7 for people like that.
Macs are far too expensive for many, but can be worth investing in for the creative suites which are the industry standard in many lines of work.
Chromium is looking to be an interesting addition to the list of OS's, from what I hear it's based almost entirely in RAM, and is geared towards netbook users (though I'm not 100% sure about the ram part) as a simple lightweight solution.
Like Linux it has been made open source so you can add whatever tweaks you may know how to do.
Being able to make tweaks like this is of course the defining thing for Linux as a whole, but you do have to have a lot of knowledge in programming to do it yourself without risking important files and whatnot.
But having said that there are dedicated communities out there making tweaks to the linux kernel (base) that would suit almost anybody, you just have to get the right distribution (ie Ubuntu for home users, Red Hat for enterprise models etc)