The biggest operational difference is that version II has a super-rotator style design that allows the tilt and shift axis to be varied at any point relative to each other. Version I allowed the tilt and shift axes to be either aligned or set at 90 degrees to each other, and changing between the two settings required partially dismantling the lens! With version II there's just a knob to control the relative angles of tilt and shift.

The other main difference is the optics, version one of the 24mm tilt shift is 22 years old now and the new version was designed for digital and features a more sophisticated design (16 elements vs 11), with modern coatings which bring a host of optical improvements. Extreme corners feature much improved contrast and sharpness, with less lateral chromatic aberration.

Alongside corner improvements the image circle is significantly larger (67.2mm compared to 58.6mm) allowing greater shifts and reduced vignetting. Maximum tilt is increased marginally to 8.5 degrees from 8. There is also a switch on the lens to lock tilt in the neutral position if you only want to shift.

You'll notice the differences if you do a lot of shifts. If you're mostly using the tilt function then version I is a good buy, though you might want to consider the Samyang 24mm tilt-shift which is a similar price but offers a super-rotator design. Performance is pretty bad wide open, but if you intend on using the lens to extend depth of field then it may perform better than the old Canon in the corners at f/8.