I have a Trackimo. I've now used it on 5 flights.
It has an indicator light that tells you if you are connected to a cellular network and another indicator light if you are receiving gps.
I use the Trackimo website on my Ipad to follow the Trackimo. The fastest update rate is once a minute, so it is really only for recovering your rocket. You won't get a flight path or an altitude out of it.
It has successfully worked on 4 of the 5 flights I've used it on. On the second flight my rocket drifted out of sight. By looking at the Trackimo website, I could tell it had landed close to a road on the other side of the field, so we took the car and drove over there. That saved a lot of time walking. We drove right to the dot on the map, and there was the rocket hanging from a tree just off the road. It would have been tough to find without the tracker.
On the one flight were it didn't work, the tracker updated when I moved the rocket to the launch pad, but that was the last update. I don't know what happened or why it never updated again. That same day I used it on a later flight and it worked fine.
One thing that I don't like is that you don't seem to get an update if it's not moved since the last update, so that kind of leaves you wondering if its working or not until it moves. I haven't spent a lot of time with it, so there may be some setting that I could change.
It's fairly small, but I wish it was even smaller. I put it in a small plastic bag and wrap it in a small nomex blanket. Then I wire tie it to the eye bolt in the nose cone. There's really not much else to do with it.
It's pretty simple to use right out of the box. About a year ago, I looked into other cell phone based GPS trackers and they all had monthly costs of $20-30. That was too high for me, so I passed on those. The Trackimo is free for the first year and then $5 month after. I figured I could afford that.
As fast as things change, I'm sure there will be a better/smaller gps tracker next year. I don't think Trackimo is perfect, but it's pretty good.