I just bought one. I've been using a therapy grade recumbent bike for years but after my knee replacement it was starting to bother my knees.
I did try out the NuStep at a physical therapy center first. The differences between it and a bike are that you get a total body workout in one machine. I watch tv series on DVD while I exercise and time flies by.
I did try to incorporate weights to my old bike routine but it got too clunky and I felt really uncoordinated. I also ended up with sore arms because of improper form. What was once an easy, enjoyable way to torch calories became a constant watch of what to do when.
The NuStep is definitely pricey. However, since I'm limited to a seated elliptical or a bike at the gym, I used the gym membership savings toward the machine. I love having it at home, I find myself hopping on it, putting two episodes of The Big Bang Theory in and my workout is done faster than if I'd had to get ready for the gym, drive, park, get set up, work out and get home.
I also love the computerized workouts (I bought the T5XR). They allow me to start at a beginner fitness level and work all the way up to pro athlete level. It's a goal I want to achieve.
I despise exercising. The bike I had (ProForm VR900 EKG) made it easy and enjoyable, but the NuStep has definiely ramped it up, without taking away the enjoyable factor.
Like I said, it's pricey but with their warranty and how well they're made I can see this thing lasting for 10 years or more.