I looked at a few cellular based designs (Tagg and Pocketfinder being the two most popular) and decided on a Pocketfinder, which I've had now for about 7 months.
Here's my thoughts on it and some reasons I chose it over the Tagg.
Both of these devices use cellular communication to report the trackers location and are designed to fit on a collar. Tagg works on Verizon's network and PocketFinder uses AT&Ts network. I have good coverage from both in the areas I frequent so network wasn't a concern to me but may be for you.
Pocketfinder is a small plastic puck, about the size of two oreo cookies stacked on top of each other. It is completely sealed and very durable. I would leave it on a dog that's swimming and wouldn't be too concerned if I accidentally threw it 30 yards or so on pavement... it seems pretty bulletproof. It comes with a black nylon pouch that attaches to the collar and holds the device.
Tagg is a bit slimmer but wider so it doesn't stick out as far from the collar but does extend along the length of the collar due to a set of wing shaped rubber antenna. It uses some sort of plastic do-hickey that attaches to the collar and then the device clips onto it. I've not examined it closely but have read numerous reports of dogs losing the device when it popped off the base or when another dog decided to give the device a bit of a chew. This was my primary reason for deciding against the Tagg system.
Both have the ability to track via web site and via mobile app for Android and iOS. I've only used the Pocketfinder site/app and have been happy with it. Currently I've got it configured to alert me via text message if my dog leaves my home and when she returns home. I am also alerted when she begins travelling more than 25mph (she's in a car I'd assume). It can be configured with multiple areas so you could set up an alert if they arrived/left daycare or a trainer and can set the areas to be large or small diameters. You can also configure how often the device reports its location. The frequency directly affects battery life.
The way I've got frequency configured, the device checks in every 5 minutes. When it checks in it reports the current location and determines whether I've altered it's reporting frequency. This way, if I set it to realtime location reporting it will switch to this mode within 5 minutes. I can also alter it to check in less often but I don't see a reason.
I get about 1 full day of use with this setting. When I put my cell phone on it's charger at night, I put the pocketfinder on it's dock as well.
I think Tagg had some neat features that tied in to it's charger/base station. I didn't get to play with that but believe it reports to the base station directly when it's in range and switches over to cellular when it gets out of range.
Pricing was similar, with initial cost being about $150 and a monthly cost (to cover the cellular service) of about $10-15. Tagg did seem like a better deal for multiple devices since they let you add another for $5 and Pocketfinder didn't have any quantity discount pricing.
These devices vary a bit from the Garmin style stuff. I believe a lot of those systems are a collar device that reports back directly to a handset (a bit like an e-collar since it has two parts). Those don't seem to have any recurring costs but I'd suspect the range is limited since they're not relaying off of cell towers.
When I had to travel and left my dog with a sitter, it really gave me a good piece of mind to be able to look and see where she was. It is also comforting to know that, if she were to wander or get snatched up somehow, I could see where she's been and where she's going. (it does keep a historic record of where the tracker has gone)
I'm sure I've left some unanswered questions regarding the solution I'm using so if you've got any questions, ask away
Bryan