I used one a few years ago. Results were mixed. The seam, which I was suspicious of, held up fine but the end split after about 18 months. Unfortunatly, I didn't spot it and, over the winter, all the grease escaped and all the salty water got in. I have just had to replace the inner joint as a result. My conclusion is that they are an acceptable short term bodge to avoid dismantling but if you have any other related jobs, I recomend a proper replacement.
For me, replacing the joints wasn't too big a problem, (the wheel bearing was a different story) two bolts holding the stut onto the hub carrier and possibly the pinch bolt holding the carrier onto the wishbone. Then, if you carefully peel back the boot on the outer you should be able to get to the circlip and remove the outer from the shaft leaving the joint on the hub/bearing. Some people have complained that getting the six allens off the inner is tricky but mine came staight off.
GTV and Racing Driver