I've had to dig up two in the past year. Both times required renting a Terra-Mite and ripping up my lawn and cost $300 before I was through.
My electrician says they work if installed properly, but they look pretty shaky to me. When I have to make an underground splice I use a 1/2 or 3/4 C-box, run the wires through the holes, twist and solder the copper, and screw a wire nut on it. Then stuff the wires back in the box, tape the ends, and pour liquid epoxy in it. It's cheaper than these commercial connectors and more foolproof.