Has anyone used any of these modern gps trackers as an anti-theft device? I'm more concerned about being able to find a bike that is stolen from my house or car than I am from a bike locked up. So when I see some of the existing products or kickstarter campaigns, they have the obvious problem of only being useful if you become immediately aware of the problem and are essentially on location - they seem very easy to remove. So I've got some ideas about how to combat that, but I thought I would check with others who may have been down this road already.

I've never used one, but it occurs to me that in order to reliably transmit its location, the GPS device will need a pretty good power source. Where does that power come from, and how long does it last?

The SpyBike products seem like a clever approach by disguising the GPS tracker as regular bicycle components. Of course this only works until the bike thieves catch on. But might be worthwhile until then.
Spybike
I suppose they minimize the power requirement by only waking up periodically to determine and transmit the location.

What if someone "nicks" the light? Those always seem to be the first thing stolen. I guess it could tell you where it is, so that's a start.

Darn lights get taken so fast, happened to my son, seconds, mere seconds.

They are all internal battery sources. Some are replaceable and some are rechargeable. According to specs I've read, battery life appears to be "months", but fortunately most have a low battery signal they send. The bigger problem is that they need to use the cell network to send back. This can be defeated if you know what you are doing and suspect a tracker is embedded. Because of the cell network connection, most of the products are subscription based, which makes them impractical for someone with a fleet of bikes. One however, LightBug, appears to have a slightly more innovative business model where they only charge a fee if you ask for too many location checks over a period of time.

You're absolutely right, IMO, to be more worried about theft from a car/house than a properly locked bike left for a reasonable period of time. If I had one super valuable bike, I might consider this approach...but I have a fleet. I rely on two things for protection:

1. Security (iron bars on back door, cylinders in the Windows, cameras).
2. Insurance.

We had a break in a few years back....they reached over the Vanilla frame, left the de rosa Titanio, Marnati, etc. and took the $600 TV, a 5 year old lap top and some dumb stuff. I guess just hope they don't know bikes!