It's ok, but is really much better suited to a blockier dog. With a slimmer dog such as a border collie I would go with a siwash style harness. Instead of one strap of webbing that goes across the chest, it has a Y-shaped front so that two straps come down and meet in the center. This style harness allows for better range of motion in the shoulder, whereas the parade style harness you have pictured can restrict movement, particularly if you are going over uneven terrain.
Edited by AinaWGSD - 9/20/11 at 11:10am
There are lots of places you can order them. I've seen them on Ebay a lot too. I got my first one from Nordkyn Outfitters (www.nordkyn.com), and I made the second one as my younger dog still had the potential to fill out a bit more at the time and I wanted a training harness that didn't cost too much to start him out on. Turns out the one I made worked just fine for the light draft work I've done with him. But the one feature of the harness I ordered from Nordkyn that I haven't seen on a lot of harnesses from other places is that the shaft loops (the loops of webbing that go over the cart shafts that help the dog steer left or right and push back against the brakes to stop the cart) are attached to the rest of the harness with quick-release buckles. This means you can put the shaft loops on the shaft first, then get the dog arranged between the shafts, and then attach the shafts to the harness. It makes harnessing go much faster, and is less scary for many new dogs. It also means that you can unhitch the dog more quickly if you need to in an emergency.
If you have a lot of uneven land, or are not very experienced building carts, you may want to start with a wagon and make a set of shafts. I find the wagon much better for heavier loads as you don't have to worry about balancing it just right and it is more stable. A wagon is less manuverable than a two-wheeled cart, but otherwise there aren't many downsides. My first rig was a radio flyer wagon with home-made PVC shafts. Worked great until we moved out and the people moving in came in early to clean the house and even though we told them explicitely not to touch anything in the garage they went ahead and cleaned it out too and decided my shafts were gargabe and broke them to fit them in the garbage can. My second rig is a nice little two wheeled cart I made out of an old milk crate and some bicycle wheels. The balance is a little off, but it works perfectly for light loads and can even carry heavier loads as long as I keep my dog to a walk. At the higher speeds it really starts to wobble, especially with heavier loads. We used it to carry stuff home from the farmer's market until they banned dogs.