I use a TK102b. It's cheap and simple and it works. You just put a pre-paid sim-card into it and you can track it by phoning the number. It sends a text message back with it's location.
The only downside is that you can't track it if it doesn't have phone reception. There is a way to set it up using gprs data to send its location to a server, but I'm not sure if it does that constantly or not, and if you can then see the last known location after it has lost reception. For this reason, I'm planning to upgrade to something a bit better. The TK102b was grand on my Deauville, but my GS is more valuable, so it's worth spending more on security for it.
Can thieves find a GPS tracker on a bike? I'm sure they can, but if it's placed somewhere hard to access, such as under the tank, it will take them time and tools to get at it. Most bike thieves lift a bike into a van. If they take it to a different location and find the tracker there, a proper tracker will show you where the bike has been, so you have some chance of recovering it.