I have some experience with these sorts of devices
Keep in mind that they typically provide you two kinds of data, GPS, and Assisted GPS. Straight GPS is when the device can get a satellite signal, and AGPS is when they're someplace that they can't see the sky, but can get cell reception (most will list the kind of cell platform they use, i.e. GSM, etc). If you're getting AGPS data, they can be off by a long ways (mile or so) because the device is triangulating it's position based upon cell tower response time, and sector data that the tower provides (that's ridiculously simplified, but close enough). That happens in places like open parking garages pretty regularly. Still, for the most part, they're pretty amazing. A modern tracker doesn't even have to see the sky directly, they can work off signals that bounce off pavement (in other words, the tracker can be hidden, and still work perfectly well as long as it can see pavement, or see through a plastic bumper cover, etc.
One commercial system I was impressed with is called a GL-200, and you can even buy them on Amazon! I'll link to one of the bigger retailers at the end. I'm not sure what they charge on a monthly basis, but the micro tracker goes for around $100-125, can accept a hard wire power adapter, and has an activation cost around $50-60.
The nice thing about these new trackers is that you can monitor them from all sorts of devices like smart phones, or tablets, and you can program them to report things based upon your preferences. In other words, you can set up a geo fence that will alert you to the tracker crossing a certain line you've set on the map, or entering/leaving a defined area (circle, box, etc), of if the tracker stops moving for more than a set amount of time....i.e. they get a 30min lunch, and the tracker doesn't move for 1.5hrs.
I've considered hard-wiring one of these into my vehicles just in case they get stolen. Since you own the vehicle, you can put a tracker on it, and not worry about any legal implications. Employees have no expectation of privacy in how they operate equipment you provide. Here's a link to some GL-200 info:
Micro Tracker Miniature GPS Tracker