They work quite well and react instantly - if GSM is available.
The problem is how to set up the device: An SMS upon any move? Upon motor-start? Within a geographic range?
Whatever you configure, you'll get either too little or too much SMS. And depending on your tariff that might cost some dollars.
Some trackers offer a gateway so you can just follow the car on a map. But this usually costs a monthly or annual fee. As a theft-protection it were a good choice to set the maximum message triggers possible and to arm the tracker if you leave the car and disarm it if you start it. This needs a hidden power switch for the device, or, if it has a bluetooth interface or the like, a functioning mobile device.
With a bluetooth or wlan interface, it is easy to detect for thieves, however.
A good GPS signal reception is key, but within a metal frame might be difficult to achieve. So you need to select carefully where to place it.