Never leave your bike unattended, ever. If you have to walk away from it, be sure it's properly secured with a Kryptonite Evolution Mini U-Lock. The Evolution Mini is the top choice of bike messengers worldwide for its portability and strength. Don't leave home without it!
There are no five star locks out there because nobody has applied that dead sexy pick-proof technology that's floating out there. That being considered, this is a great lock. Pair it with a Pinhead lock kit and you'll have a great urban setup for safe storage as long as you're not up against a skilled thief with a lock pick. It's also an improvement over the old barrel locks that could be picked with a Bic pen tube. Wow. Somebody got fired over that one.
This lock is perfect for running through the rear wheel around a bike rack. It's not a beast to carry and it can fit in your back pocket for quick jaunts if you're not carrying a pannier or backpack. I prefer this smaller lock after years of dragging around the longer shackle version. Like every other U-lock, the shackle eventually gets "sticky" from rust. A wipe down with some WD40 and application of grease restores them in quick order for another year or two of trouble-free use.