I purchased this tiny lock along with a cable (see link below) for my bicycle, just to secure my saddle and rack-top trunk bag. It's perfect for this purpose. Following the link to the cable, I'll just attach the review I wrote for that item since it also covers everything I care to say about this lock.

http://www.amazon.com/gp/product/B00165PA3A/ref=oh_details_o00_s00_i02

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I wanted something that would secure my saddle (which is sitting on top of a seat post with a quick-release adjustment) and rack-top trunk bag (which is secured to my rack only with velcro straps) against casual theft, and which I could leave on all the time. This cable is the exact length I needed and threads through my saddle rails, a bit of my bike frame, a bit of my rack, and a loop on my trunk just perfectly. The entire cable is covered with a soft material to prevent scratching of the finish on the bike. The price is right and the quality of manufacture seems good.

I am using this inexpensive, small, light lock with this cable:
http://www.amazon.com/gp/product/B001M0NZ7M/ref=oh_details_o00_s00_i00

The cable doesn't rattle against my frame or the rack thanks to the soft covering, and by positioning the lock in the right spot, snug against my soft trunk bag, it doesn't rattle either. When riding, there's nothing to notice: Not the imperceptible bit of extra weight, no extra wind resistance, no noise, and it isn't immediately visible either since the cable is mostly hidden between my saddle and my trunk.

Obviously this cable is useless against any thief with basic tools, even something as simple as a hand-held wire cutter. As the cable's own package states, it's to prevent "grab and run" attacks. As always, security and convenience are inversely proportional. If you want better security, you need a thicker/heavier cable with a larger/heavier lock and quite possibly will have to string it up and break it down each time you lock/unlock your bike. For me, this cable hits the sweet spot; leaving it on all the time adds barely any weight, plus I don't have to think about locking or unlocking it when I secure my bicycle. I feel much more confident that my saddle and bag will be there when I get back to the bike after a stop into the grocery store or movie theater.

This is the second Avenir product I have purchased with a silly typo on the packaging. (It's "anti" not "aniti" - Avenir, I'm available for proofreading and editing!) Other than that, I have zero issues with this product. I'm going to buy more of them, and more of those locks, for my other bikes.