I used a borrowed GTAx 550 for a couple of months this summer. After a couple months swinging a Discovery 3300, the GTAx 550 seemed significantly heavier.
The controls and display on the 550 are easy to use, but I wasn't really impressed with the pinpoint feature. I frequently had to try a few times to get it to work. The 550 found several coins that my 3300 had missed. I found the targed I.D. to be very accurate, but I didn't find coins as deep as with the 3300.
The arm cuff on the GTA is too wide and isn't adjustable, so it would slap my arm at the end of each swing. It does have a battery pack with a belt clip, but I found that leaving it on the machine was more efficient, even with the greater weight. Taking it off changed the center of gravity and I didn't like the power cord stretching from my waist to the machine.
Overall I liked it and I don't think you would be disappointed with it. Were I to buy a new machine today, I would probably look elswhere, but wouldn't pass up a good deal on this one.