I had PF about 3 years ago and orthotics were the recommended treatment - plus to avoid doing anything that hurt - duh!
Takes a long, long time to heal.
Anyway, my sister has a gazelle and uses it pretty faithfully. It is a decent cardio exercise if you make yourself work hard enough. Have you tried an elliptical or cross-trainer. There is no impact on your feet on these and you get a good workout - even with bad knees (again, because there is no impact involved). On the con side though, since there is no impact, neither is a true "weight-bearing" excercise like walking or running which you should have to help prevent osteoporosis.