For my new city bike I'm thinking of getting a U-lock. Price of the bike is around $900, so I probably don't way to spend more than 10% of its price on the lock.
So far, Kryptonite locks look pretty good, altough that may be due to hype. Their "yellow series" locks seem pretty safe, but I doubt I would lug around a 2 kilo beast for long. The mini series lock looks nice too (and cheaper!) with its 1 kilo of weight. But is it secure enough? Are there are (perhaps less hyped, but good) brands that I should consider?
Note that I won't be parking my bike on the street for too long. I just want to be able to pop into a store without being on a paranoid lookout for my bike every second of my grocery shopping. Is going for the more expensive lock overkill in such scenario?