I've always thought of the bike alarms as more of an idiot deterrent than a theft deterrent/prevention measure; if a skilled & confident thief wants the bike, they'll get it, that's what theft insurance is for. But at least it stops kids etc from sitting on/playing with the bike etc, which I used to catch people doing all the time prior to getting the alarm.
All that said, I've come across an issue with my current unit; for whatever reason, the Tilt sensor likes to randomly go off and it freaked me out the first few times it happened. I was sure someone was messing with my bike at night, but I've since witnessed the alarm trip for no apparent reason on numerous occasions. Possibly a defective unit, as I've also had the Proximity sensor chirp for no apparent reason at times too.
Bottom line, having anything is better than nothing, if only for the added peace of mind -- just make sure it's not your only theft protection, ALWAYS have theft coverage on your insurance plan...