My original post about Mavericks and MP150 can be found here: http://www.igorkromin.net/index.php/2013/11/01/canon-pixma-mp150-scanning-no-longer-working-on-os-x-mavericks/.
This is the issue that the MP Navigator is having. The 'Save' and 'Send' buttons are misaligned and not clickable. This 'Scan' button works but there is no way to save what you've just scanned.
There is a workaround to this.
The 'Scan with Scanner Driver' button can be used to open an alternative scanner interface.
This presents the window below that lets you scan, save, etc. The quality of the produced images is noticeably worse than the scanner used to produce under Mavericks however.
Looks like the MP150 will have to be replaced soon with a newer device.