Our dev server is running 5 rails apps in testing on a 256 box so it's well worth the money if you just want to try it out (it's about 20 bucks a month).
They also gave me a sweet tshirt. But that doesn't skew my love for the RS cloud ;)
1) Create an amazon S3 bucks 2) Upload your files to that bucket 3) Publish that S3 bucket to cloudfront with two clicks in the AWS console 4) Update your DNS CName to point to your CloudFront distribution
Rackspace is fantastic for customer service, but for CDN services, you'll have to weight whether the difference between 22 cents/GB vs Amazon's 10 cents/GB for US bandwidth distribution is worth it.
IMHO it is tough to justify 12 cents/GB just for "better" customer service for a CDN.
Amazon CloudFront uses Amazon S3 as the origin server to store the original, definitive versions of your files. Normal fees will apply for Amazon S3 usage, including "origin fetches" - data transferred from Amazon S3 to edge locations.
Amazon S3 rocks! We've moved everything over to them - except a billion emails still sitting with Rackspace...