I recently had to step in as the lead ColdFusion programmer on a project while my co-worker was on vacation. Apparently, I did a good job as this was what was sent to my boss:
"Ben has been such a tremendous help in Martin's absence. Just wanted to let you know how much we appreciate his professionalism and prompt attention to the post-launch issues with which we are dealing right now. He had some big shoes to fill but he is doing an admirable job! So thanks! You might want to give him hazard pay .. We are a rough bunch!"
Client-appreciation is enough to make it all worth while... but the "hazard pay" certainly wouldn't hurt ;)