This is a fantastic i2c 16 channel io port expander with level shifting. The only things I would suggest is to re-layout the boards i2c pins and add thru holes on the corners so standoffs could be used if wanted.
The i2c pin layout I am most familiar with is VCC, GND, SCL, SDA then you can have the Int, RST, and the oscillator on another 3 pins. This way a 4 pin and a 3 pin right angle headers could be used for use on a breadboard or with standoffs. (just cut down the 6 pin rt angle header that sparkfun sells)
If you also add the 4 pin 12c pads to the other side of the board it can be used in a chain of i2c devices.
Please consider re-laying out this board, So far I have not found any i2c port expander that comes close to the SX1509 functionality and features.