Dont bother with an alarm. Waste of time and money . Flatten yer battery and nobody ever takes any notice of them .
Persil has a point about them flattening the battery- but if you get a good one, then that's unlikely to happen if you're commuting on it. A cheap one probably will kill your battery because they draw more current and the generator on a 125 won't be all that great.
As for nobody taking any notice of them, well that's true. But you will, and the theif will. If you're within earshot of your bike at work, or there is somebody who will notice, like a security guard, then its good for peace of mind. And it'll immobilise and at least make it difficult for them.
Rilot said:
Go with a really big chain. Pay good money for it. £100ish.
BIKE mag had a test on a few months back and rated an oxford hardcore (£50) as the best in test. It lasted a whole 3 minutes.
I think a reasonable chain to stop opportunists and ****** with vans, a couple of cheapish (but still sold secure @£25 each) ground anchors, an alarm/immobiliser (datatool system 4 @£250 fitted) and datatag (£25) and you'll have a better than average security set up that would put most people off.
Things to avoid IMO are:
Cheap ebay alarms - Draw too much current and can be fitted (and therefore un-fitted) by anyone
Disc Locks - Steering lock and alarm will stop it being pushed away, annoying hassle when you're only stopping for a few mins.
Home-made modifications ie, secret cut-out switches - Would you buy one with these?