If they are those things that you have to attach to the bike so they'll activate, then I think they are a waste of money.
All you have to do is park up, get off the bike thinking about something (e.g. the reason you've parked up) walk away forgetting about the attachment and it'll activate.
Like others have said, decent gear helps, but the best safety device on the bike is you.
BTW, welcome to the site, and the best bike money can buy
