I have a Tri-Tronics Sport G3 (I'm 80% sure that's the model name). Just tried looking it up and it looks like they no longer make it. It looks the Tri-Tronics Sport Pro is now the comparable new model to it. Anyways, I bought in the Fall of 2012 and have used it A LOT since then and have had zero problems with it. Works reliably every time, the batteries hold a good charge. If I had to buy one over, I would buy the exact same thing.
As always, remember to train your dogs to the collar. Don't just zap them to zap them and assume that they know what it means. Teach them how to turn off the zap. Put the collar on a very low setting, and hit it continuous, then give the command to the dog. Once the dog does the command, then release the button. Thus, the dog learns how to turn off the negative pressure that they are feeling. Anyways, there's much more to it than this. Enough that an entire book or DVD series could be made on it, much more than a forum post.
I have a tone (beep) on my collar. And I have really enjoyed that function. I used to use a whistle to communicate to my dog in the field, 1 whistle meaning stop and sit, 3 whistles meaning come. Since I am public land hunter, I always want to be more quiet and hate talking/whistling to my dog in the field. We've now got it down to where I can use the tone on the collar to mean the same as the whistle, and now we are one quiet duo in the field.