Well aware that DLR is here!! I have recently reading up on all of these and was wondering if there were any specific benefits of using one language over another?
For example performance benefits! and available functionality through standard libaries!!
The answer is "depends".
F# is great if you need to functional programming is used extensively by Academics that need its pure computing power to get something done.
IronPython and IronRuby is great for being able to create applications that run on the CLR because they give you the .NET goodness with the speed of writing Python or Ruby. I don't think that any of these is more preferable to another without it being in a proper context