I finally started using Amazon's EC2 yesterday. I've been meaning to learn it forever, but assumed it would be time-consuming to get registered, set up an instance, and so on.
Not true. Thursday morning at 10am, I registered for EC2. By 10:30 I had an instance of Drew Conway's Py/R AMI up and running, with several additional libraries installed, and a few GB of data I wanted to crunch uploaded to the server. Very fast turnaround.
Eight hours and $4.77 later, I'd crunched a lot of numbers -- by far my most productive workflow all week. Highly recommend it.