A schedule performance index greater than 1 means that the project is ahead of schedule, which is, of course, a good thing.
However, an SPI that is way above 1 may mean that the project manager has added a lot of padding to his schedule, which is not a good thing for the company (The project manager reserved some precious resources for 6 months while he only needed them for 3 months).
Now when you have an SPI that is larger than 1, it is very important to make sure that all the remaining tasks get finished withing their allocated timeframe, and it is important to ensure that your resources still feel the sense of urgency. Remember that in the beginning of the project, an SPI bigger than 1 is pretty standard and doesn't really mean anything. If you maintain an SPI greater than 1 throughout the project, then this means you are really doing a good job (provided, of course, the Schedule Performance Index is not way above 1).