as a microsoft developer interested in looking at ruby on rails early on, i loved when the sapphire project was first released. i had played with earlier version and after re-paving my machine need to take a look at the updated versions.
if you don't own a copy of visual studio 2008, when you get the ruby in steel text edition, you get visual studio 2008 (ruby language support only) -- so this isn't an add-on only, it is a full deal. if you already have vs2008, this will install into your current environment. it has all the goodness you'd expect of an IDE: color coding, snippets, code folding, integrated debugger, and interactive consoles.
the sapphire guys also have the developer edition which is a little more full featured, and includes a faster debugger and few other features like dynamic update intellisense. DHH was quoted as saying:
"Ruby In Steel is particularly interesting to developers coming over from the Microsoft world as Visual Studio is a familiar environment to them. That'll likely ease the transition." source: InfoWorld
i think this is great for people interested in learning new environments as well as new languages. already being familiar with a tool like visual studio may help you understand about ruby without having to learn any new tool.
and no, it doesn't run on a mac.