Hi, They are good if you get one that has resistance with it. The one I just got is Life gear walker by Brenda DyGraf, it also came with two wrist weights and a workout video. Although you really don't need a video. I use mine in front of the TV and watch one of my favorite shows that way I won't get bored!
Depending on how fast you go and if you use your arms or bend your knees sometimes, you will get a pretty good workout. These kinds of machines are good for people who may have joint problems too. The Nordic track put too much stress on my knees which is why I went to something like an air walker.