Many popular bicycle locks can be opened with a Bic pen

Many commonly sold bicycle locks that use the round-style keys can be opened with a common Bic pen barrel in less than 30 seconds. This includes many Kryptonite U-Locks. But any lock of any brand that uses the round key is susceptible to this type of attack. There is a lengthy thread on the subject at BikeForums.net:
My jaw dropped when I saw this at first. I tried it on my new New York Chain (5') with EV disk lock. The lock has a round tumbler . . . After a little fiddling I opened it with NO problem. I was so upset I rode right to my local bike shop (Westchester Pro Bike Shop, Westchester Square, Bronx, NY) and showed them my BIC key.

I used the universal BIC key to open up one of the shop's U locks that were on the wall. Needless to say, that opened. The owner and store mechanics were as upset as I was and the customers around me.
You can see several videos of people easily unlocking several different types of circular-keyed locks linked from Bicycle Forums.

Any type of lock can be picked, cut, or broken by someone with the right skills, knowledge, and tools. But the fact that the locks with circular keys can be picked using an extremely common and inexpensive tool, and that the knowledge of this is now spreading like wildfire on the internet, seems to make this type of bike lock a very poor choice now.

If you want the best kind of security for your bike, look at the bike lockup advice from the New York Bicycle Messenger Associate or this article on bicycle lockup from T. A. Magazine.