I was just gonna start a Treadclimber thread!! I've used all three. I used to own a treadmill (gave it away), used the elliptical religiously at my gym, and bought myself a Treadclimber a few years ago.
I'm not a runner, so even though I used my treadmill daily at one point and was able to build up speed and distance with it, my heart was never in it and it was too easy for me to stop. I eventually gave it away to a friend of mine.
For several months I used an elliptical at my gym every night for my cardio. I was able to really build up the resistance and tried variations with intervals, but after a few months I think I hit a plateau. I mean, I wasn't gaining weight, but I wasn't losing any either. I was doing AN HOUR of elliptical every night and not losing a single pound. After a few weeks of that, I said forget it.
A few years ago, I bought myself the Bowflex Treadclimber. It was something I wanted for years, but didn't have the money for. So I saved up for it one year and got it as a present for myself. (FYI, it gets delivered in several boxes that weigh a ton each, so I needed an ex to put it together for me.) I used it, then stopped. I put off any kind of workouts for a couple of years and just kept packing on the pounds. Eventually, I found my way back to it last month. I've been using it every evening after work. It's definitely a workout. I think out of all three machines, I sweat the most on the Treadclimber. You can feel the burn within the first 5 minutes. At the resistance/level I'm currently at, I burn about 500 calories in 40 minutes a night. I'm trying to build up to 700.
If you have the money for one, I think you'd like the Treadclimber. They say in the commercials it feels like walking on sand, but it's more like slogging your way up an uneven mountain. You can feel it in your thighs and glutes. The movement isn't as fluid as an elliptical, but it just feels more effective. It feels like running, only slower, and much much better on your knees and shins.