On my trek interchange bags, i like how reflective-d up they are, and how they can go from nearly flat to carry a full grocery bag's worth of groceries. I like the zip-away rain jacket they come with (even though they are pretty water-proof /no wet inside yet), and the zip-away adjustable helmet net. I also enjoy the padded handles, that feel good even if there is 25 lbs of stuff in the bag.
While i like that they have water bottle nets on the aft (away from pedal) side, i dunno how much use i'll get. I also like all the loops on the outside here and there, though ive yet to need them.
Most of all i love how easily i can unclip and remove them.
- Andy