Largely, for the SV, the expense of an alarm doesn't justify the protection given.
Personally, I spent £30 on an alarm, fitted it myself, and it saved the bike from being stolen once. £30 well spent. Since then, just in case I didn't hear the alarm (no-one else reacted, but I ran out of the office), I've custom made my own tracking system and fitted that to the bike. I'm always reluctant to go into detail about the tracker, but I'd estimate that cost me about £80 to build, and about an hour to install.
These days, the bike doesn't cover anywhere near the distance it used to, and when I'm at work, it's parked in a very secure location (swipe card access is required to access the bay - otherwise it's completely sealed off, and is covered in CCTV constantly). That means that I rarely use the alarm (to the extent that it's no longer plugged into the indicators), but the tracker is always active.
So, is an alarm worth the cost? Well, it depends.
