I bought and I'm using the NY K chain lock that must weigh 10 pounds. If nothing else I figure it will discourage anyone enough to find another bike. I still try only to park in safer places (attended garages) and this is all for my Cannondale. (Actually since it has prismatic paint, Shimano 7700 wheels and the name is big around here it is probably less safe than my nude Fly with tubulars.)
I use my bike to get around now so I figure it is worth the effort. When I just cycled it didn't matter all that much as I did not leave the bike very much.
The lighter cable locks are very easy to cut. I would not trust one except for a quick trip into a convenience store with the bike in sight. The best it can do is protect against an amateur snatch and grab.