I have had two different GTI 2500s over the past 3 or 4 years. Both did exactly the same so at least I can say between the two they were consistent.
With both you could never be 100% sure if you were digging a penny or a dime because sometimes it would register as the opposite. Depending on the area and my time, I just dig almost everything in the coin size range. If the area is very trashy I will notch out nickels because the pull tabs fall in that range, yeah I know, sometimes some lower conductive gold rings too. I went with a friend last month to a park, he had his Minelab Sovereign GT and I had my GTI. After 1 1/2 hours of hunting I had a pocket of coins and he had a pocket of junk. I found almost $7 in change, no jewelry. He found 13 cents and a pocket of pull tabs, and some of those pull tabs were recovered at almost 12 inches. I'm not saying the Sovereign is bad, I'm just saying that using the GTI saved me a lot of time and helped me recover a lot more coins than my friend did. Now he with the Sovereign can leave me far behind at the beach, there isn't one time that the guy goes and doesn't find something good.
I have hunted in all metal and was impressed at how deep it will go. Also when hunting in all metal, you can look at the meter to have a more or less idea of what is down there. I then use pinpoint because I don't want to be digging up a beer can at 12 inches down and destroy 3 feet of grass to get to it.
By the way, when the GTI gives you the bell tone of a coin but on the meter it shows it one notch to the left of penny, it will be almost 100% of the time a non copper penny (zinc/copper). I say almost because sometimes some junk can slip in there too. But with my experience, it never turned out to be a copper penny, dime, quarter, etc. Quarters are another one that the machine is pretty exact about. It doesn't get confused like it does with pennies and dimes. There have been a few cases where the machine said 50 cents and after digging some of the soil out of the hole it then registered as a quarter.
Good luck with your GTI. Just like the advertisement says you will dig less junk. I love mine and would never part with it again. If I could get my first one back I would apologize to it for selling it. I was one of those that just had to try out as many machines that I could. Every one I tried after I didn't like because I wanted the features that the GTI had. I know that this is my opinion and the others that post here will probably have a different one, in which we are all entitled too.