I have a couple GU's myself. I have to say they do chunk a mean mark.
I did however have to change out the pulleys, and they are Very finicky about when they'll fire a primer. Only about 20% of the time do they actually fire. GU said send them back which is nice, but I can't afford the equipt. downtime with a kennel full of dogs. Looks like most people don't have any problems though... Two other friends of mine have also had zingers (field trialer models)for several years, they fire each and every time and they leave them sitting in the field and just move them the next time they use them. Nor have they started to rust. We've had them side by side with the GU and it does look like the zinger is a little heavier duty... But so is the price. On the other hand I like that GU really strives to work with it's customers, and seems to vigorously support our sport. I also have to say though that I'm fixing to purchase 2 more wingers and I'm still unsure of which product I'll go with. The funny thing is that I have two "Strong-Arm" wingers that are no less than 10 years old with the old door-latch release, and to this day, I've never had a malfunction. They are bulky and don't fire a primer however like the newer wingers on the market...