I haven't tried out Joyent, but I've tested out Linode and Storm-on-Demand and they easily beat Amazon EC2. My benchmark was running our production image processing algorithm that utilizes OpenCV via python on Ubuntu Server (definitely CPU-limited).

FWIW, here are my benchmarks, along with pricing for Northern California regions:

Linode 512: 15.2 seconds, 65 cents per day

Storm-on-demand 1 GB: 13.05 seconds, 5 cents per hour

EC2-micro: 168 seconds, 2.5 cents per hour

EC2-small: 63 seconds, 9 cents per hour

EC2-medium (high CPU): 12.13 seconds, 18.6 cents per hour