We have two 8 y/o cats who have roamed free since they were young. But after several near-misses with cars, all-night stop-outs, and expensive vet bills from altercations with other cats, I reluctantly decided the fence was the only option. We were the laughing stock of the street and even the vetinary office who vowed it would never work, BUT IT DOES. The biggest issue was with training - the manual advises a leash or a tie-out so that if the cat jumps when "corrected", he stays on the right side of the fence. But most non-Siamese cats will not walk on a leash, and we found they wouldn't walk on a tie-out either. Eventually we put the collars on them and supervised them extremely closely, retrieving them from the other side of the fence every so often. Occasionally after an escape we had to turn the system off to allow them back in without being corrected. It took about three weeks of 2-3 hours a day and was admitedly intensive from both a time and emotional standpoint - I hated putting my cats through it and they got very down about it in the heat of the training. But gradually we began leaving them unsupervised for longer and longer, and now they remain within the boundaries of the property, even after dark. It is such a relief to be able to let them out and not worry about where they are. If you don't have the time or the patience for constant supervision during training, an alternative option is to first purchase an indoor system, such as the PetSafe Pawz Away Instant Pet Barrier. The cat learns how the system works by himself in the safety of your home, and will generalize that learning to the outdoor fence, dramatically reducing supervision time. Yes, the fence and the collars are initially expensive but the peace of mind is well worth it.
Update a year later: Although we were thrilled with the effectiveness of the fence for the first year, the more feral of our two cats became extremely depressed the following summer, lying around listless and not wanting to go outside. If we carried him outside, chipmunks would run right in front of his nose and he wouldn't give chase, even though he had been a master hunter. Since he had always been the more reliable of the two cats, we ultimately decided to take him off the system and allow him to roam free again. The depression has cleared up and he is back to his old self, although we miss the peace of mind! Meanwhile, our other cat (with many more years of domestication in his ancestry) remains contained and we've seen no changes in his personality or demeanor.