How has caffeine intake affected your work performance?

Many nurses consume caffeine in one way or another, whether it be through coffee consumption or a soda.  

For many nurses, caffeine offers positive benefits for helping them to get through a shift or two.  For others, the effects may be more negative.  

For myself, I regularly consume over 900mg of caffeine every day.  For a period of time, caffeine seemed to be helpful, but now I wonder.  Lately, I've been angrier and angrier, and am wondering if my continued climb in caffeine intake may be contributing.  I'm still getting work done, but perhaps not as well or as enthusiastically.   I'm delegating off more and more tasks that cause me any amount of stress.   I've also left work early a few days because I was too angry to complete any more work (or fatigued, or both).  

How does caffeine affect your work performance? Has anyone noted any negative effects from too much caffeine and work performance?