My first detector was the GTAx550, I got it last may. I recently purchased an AT Pro, but have yet to use it (waiting for spring). My first hunting season with the 550 was amazing, I found so much great stuff with it. I found 1300 coins, 54 silver coins (including 3 Barber halves and a Morgan silver dollar), some Indian head pennies, many wheat pennies, 7 silver rings, and a solid gold 2-piece cuff link.
I'm SO GLAD that the 550 was my first detector. It is quite easy to learn, took me 30 or 40 hours to figure out what I was doing and what to listen for. It goes fairly deep, and the pinpointing is really good. The only problem I had was that I wore out the pinpoint button and had to have the screen replaced. (I now have a ProPointer, so that should save some wear and tear on the pinpoint button).
The 550 was my first detector, so naturally I have a soft spot for it, but I do think it's a great machine regardless! I ordered the AT Pro because I'm interested in the iron audio, the places I usually hunt have a lot of iron to work through. BUT I will always keep my 550, I think it's a good detector.
You were smart to order a Garrett!