Shall I use the hardware, and time to write the driver software (or is it avialable somewhere?), and if so, what do I gain for the effeort

When hardware, as in your case, is available, I would definitely use it. Advantages:
1) you do not need to disable other interrupts for timing of the IIC
2) full speed driving the IIC bus
3) less code (it mimics a parrallel interface)

Philips has an appnote on using the 522 which in IIC respects is identical to the 66x series the code (i2c552-c.zip) (an435a.exe) etc is here for the actual appnote do a search on the appnotes as in AN-555

Ha' en sjov weekend

Erik