I don't think they're worth the money TBH.
They won't prevent a pro from nicking your bike.......they tend to either park them somewhere and watch them for a few days to see if plod pay any attention to them, or park them in a container unit or where a signal can't get out.
So that leaves the average bike strippers, or the fucktards who want to hoon around the fields for an afternoon then torch the bike.
In either case, consider if you'd actually want your bike back, semi-ruined.
I reckon physical security is a better investment.......a ground anchor and the biggest fekk-off lock and chain you can find.
Catching the scrotes and getting back the sad, wrecked remains of a stolen bike for a lot of money invested in a tracker, or stopping them from nicking it in the first place for (probably) less of an investment? Easy choice IMO.
Oh, and it only takes an hour with a decent camera to record every aspect of your bike, all the farkles, panniers, HID setups and any other bling you have on it, then pay a few quid (literally) more a year on your insurance to insure it for the value of the bike, AND the bling, all together.