Long Zheng wrote a user focused review of the benefits of ReadyBoost in Vista “ReadyBoost works, but not a miracle“. He made his assessment based on the experience of running various applications on his Toshiba M400 and the readings on the Resource Monitor.
ReadyBoost does not improve performance, it only improves responsiveness. It won’t make your system or Photoshop run any faster, but it will make things faster to load and initialize to a working-state. — Long Zheng