My mom has one that she used to watch tv every night with. The upside is you are getting some exercise, the downside is you can't really increase your intensity beyond going faster and eventually you'll cap out there. Hers also squeaked horribly after a while but I don't know if she could've fixed it with wd40 or what not.
I'd probably recommend a rebounder over a gazelle, something like this:
http://www.amazon.com/Pure-Fun-9005M.../dp/B002N5AHFS
You can get a similar workout, you can vary things and it is cheaper. I've heard of people who have had mini trampolines and have been happy with them.