Has anyone had luck with just walking as cardio?

Yep. I lost 90 pounds in 18 months with walking as my only dedicated exercise. I started out only walking 30 mins morning and night because I had plantar fasciitis, knee problems, and no stamina due to being so out of shape. But I kept adding to it... pretty soon it was an hour in the morning and another hour at night. I also walked faster as my conditioning improved. Walking is a great all over exercise. Now I even jog some!

Walking does work if you're willing to put the time into it. If you're in a hurry though, you might want to add some more vigorous activities or look at other options ;-)

Looks like you got mostly great advice. I think most of the difference is what you consider walking. If you are thinking the walking you normally do around the house or your neighborhood, then your benefits will not be anywhere near what you got from running. But if you are willing to go long and hard enough to get the heart rate up and keep it there a while, you may actually do better since you might be able to push the cardio without dealing with other pains. Several times in the past when I've had running injuries, I've switched to walking and never felt that I lost much from when I had been running. In fact, one time I'm sure I improve quite a bit during the two months I was walking.

If you really want to see what is possible, check out a race walking clinic. Some of those guys are walking 9mph or faster. Most of us can't run that fast.