I've been using derivatives of this alarm for the past 6 years. Only once was there a problem, the immobilizer failed to disconnect and the bike would not start. Was replaced under warranty and no further problems.
They do have a sleep mode to preserve battery life, but in my experience it needs to be left for some days for this to happen. Regular use and you would not know it exists. No alarm is going to stop a determined thief, but any alarm might help. As it's a freebee, why not.
