U-locks are ugly and awkward to mount on a bike. The lock companies have tried for years to find a strong, reliable alternative to them.
Heavy duty chain locks are tough enough but also next to impossible to mount on a bike. They are almost always carried in a basket, pannier bag or backpack.
A few years ago the Abus brand reinvented the folding bike lock with its Bordo model. This new Kryptonite version of that is stronger, less expensive, stores easily on the bike and looks great.
Made of 3mm thick hardened steel plates and coated with thick rubber, the Keeper 585 unfolds like origami to a larger than U-lock size and is tougher than most of the U-locks on the market.
Nothing else stores as easily or looks as good on your bike.