Rx allows you to write code that orchestrates concurrent events together. If you've ever used the TPL (i.e. Task), then had to do convoluted backflips to try to ContinueWith or WaitAll on the right things, Rx is for you.
For example, the workflow of "For each item in this array, call a web service, and when all of those requests come back, do something else. If any one of them fail, fail the whole thing".
Disclosure, Shameless plug ahead: The book that Jesse Liberty and I wrote about Rx was designed to solve exactly this question, "How do I use Rx in my day-to-day work?"; "What can I do with this?"
