I commute to work frequently on my Raleigh i8. A while back someone asked me what kind of lock do I use. I did not have a response, I had not been using one. Last year when I started commuting I would ride to offsite meetings and other events and park my old mountain bike anywhere. I did not have a care, the bike was worth nothing and looked even worse.
However, now I have upgraded and should probably take care to secure the investment. I had an old kryptonite bike lock that was very heavy and bulky. I opted not to use that but rather just buy a cheap lock. I happened to be near a store and picked up a wordlock (http://wordlock.com/bike_locks/) My intent was to just secure the bike.
A normal combination lock can be defeated without any tools in 30 seconds. http://lifehacker.com/5929247/crack-a-combination-bike-lock-in-under-30-seconds I have not tried it with my lock, but I am guessing it will work.
However, after reading a bit more about bike security and how easy it is to defeat locks, I am going to do some more research on what to purchase. They even recommend a second lock. That way you have two defenses.
http://www.commutebybike.com/2011/01/03/the-very-best-bike-locks/
It would be great to not have to use a lock. I realize it is worth it to secure both my investment and my ride home.c