Rather than further hijacking the assault bike thread, I decided to start my own.

I read this article that was linked:
http://www.bodybuilding.com/fun/10-best-and-worst-cardio-machines.html

And frankly it's bogus.* The most recent study is 15 years old and didn't even examine elliptical machines.
Read the comments to that article to get a more accurate picture.

I've used just about every cardio machine at the gym, at least once. There's a few things I will say.

The "arm-less" elliptical machines do not raise your HR as effectively- there are many different types and they are not all equal.
Perceived effort decreases when you take your hands off of the moving arms on the elliptical- even if your HR remains similar.
Recumbent bikes really do suck (I used them a lot last year), I reach a point where my legs get tired from the rate/difficulty but my HR has ever gotten above 140 BPM on a recumbent (and usually is closer to 120)

*It's bogus because it doesn't take into account what you do on it. Any cardio machine can provide difficulty by increasing resistance (or grade), or speed/rate.  
When I jog on the treadmill at 7.5 mph with a 2% grade, I'm burning about 1200 calories per hour, (according to the built-in calculations.)
When I use the elliptical maintaining a pace of "12mph" on resistance setting 13, I'm burning 1400 calories per hour (according to the built-in calculations).

Unlike a treadmill, where the machine dictates the pace to your body, on an elliptical your body dictates the pace you are going at (and is very difficult to be completely even)
I can actually push my steady-state HR higher on the elliptical than I can on the treadmill. It probably has to do with engaging the muscles in my shoulders and arms more effectively.  

You get out of it what you put into it. You can read a magazine or watch a movie on the treadmill, a stair stepper, a stairmaster, an elliptical, or almost anything. But not if you are really pushing your limits, sweating like a maniac, and huffing and puffing like a steam train.
I like the low-impact moderate to high intensity cardio I get on the elliptical. I use it about 2x a week along with a bunch of other stuff.

I don't like upright exercise bikes because I find them to be extremely uncomfortable, much more so than riding an actual bike.

The variable stride models are the best....by far.

Short choppy fast steps, or long strides. Makes the workout much more effective IMO.