Recently I have been experimenting a lot with I2C. My A.I. project uses the DS1307 RTC and a work project uses several I2C ADC's.
Normally the Arduino Wire library works fine with the DS1307 RTC but it does not play well with 3.3V @ 8MHz devices such as my Micro Magician and does not support fast mode (400KHz). It also has some compatibility issues with some devices such as my ADC's because it does not support a repeated start condition.