Hiya,
I haven't tried one of the alarms mentioned above, I purchased this little neat device that I found on the web.
www.timani-ltd.com This is such a great device... but still got a nice big chain lock ;-). Cheap as well! Had my bike stolen before and its such a pain. Hopefully just having something else other then a lock will make people think twice and go for the next bike. I don't want any1 to have there bikes stolen but at the end of the day it happens.