In the last ten years I've never see an OE imobiliser fail, but I've seen lots and lots of aftermarkets stuff go. And like everyone else's observations, alarms only work if you can hear and identify it - then you need the balls to do something about it!

Ye' ole' thieving scumbag may on the other had be put off by a screaming bike, so in some cases it might work. You need to balance this out against any hassle the thing is going to cause you.

I have an alarmed disc-lock, sure it's not going to stop a determined thief, or a man-in-a-van attack. It's not loud, but it's so dammed high frequency that re-setting with the key is a task that leaves your ears bleeding, never mind trying to defeat it, or drag the bike away with it. But it's only a deterant and at least if it fails I can ditch the battery's and carry on using it as a lock! It also goes off at the first movement, this can be a double edged sword at times!

I too have dispensed with the big selling chains and locks, go for a mixture of things, If you can buy it, then so can a thief, then figure out how to defeat it. The best chain I ever had is a piece of lifting chain my old man got for me, it took me 10 minutes to cut it down to size in a workshop with good cutting gear. I figure it gives the bike 10 minutes of noise - maybe someone will notice, maybe not.

I also, use some steel wire 'slings' (for lifting steel), thinish and quite light. Cuting with a grinder is difficult (as the strands snag the blade), bolt cutters don't work too well, but you can get through it with good quality wire cutters with some time. It's not going to stop anyone, but it will slow them down, and hopefully they'll move on

I work on the assumption that if my bike is a bigger pain in the ass than the one next to it, then it's a bit safer. Hey, if I go to the effort then I don't feel bad about the bloke next door who's bikes gone but got into his room 5 minutes sooner!

Back to the topic, I don't think Alarms themselves should be singled out as problems, but anything electronic can go wrong - so the more basic it is, the easier it is to repair, fix, ripp off or bypass on the road.