My personal take is that exercise physiology has come a long way since Jane fonda. I think she does some really unsafe stuff but i am sure a MOD will jump in on that. i would rather hear you put on your sneakers and went out for a walk.
If your dog is fat, you aren't getting enough exercise
I agree with Laura - it's not so much the specific activity but whether you're doing something that safely gets the heart rate up that matters. I do have an older Jane Fonda tape, and I enjoyed it. I don't really have the space to do aerobics routines anymore, but what worked best for me in the past was to do a different tape each day or each week. Mixing it up really kept me from getting bored.