I have one, but I haven't had the chance to use it very much. We bought it just before we moved and I used it a couple times and we just got it set up in our new house and I've only used it once.
I have heard a lot of people that have used it and lost lots of weight. Of course, these people were really out of shape to begin with, so any movement at all would probably have given them the same results.
When I use it, it doesn't seem nearly as difficult as the elliptical at the gym, but I definitely think it's better than nothing on my off from gym days. Probably about as effective as walking, would be my guess, just from the feel of it.
If you lean forward when doing it, it seems to work out your arms/upper body a lot more.
I don't really have room for a treadmill or a bigger elliptical, so I figure the Gazelle is better than nothing.