I don't have one yet, but I'm getting closer to pulling the trigger on it. I assume you're asking because you've used stationary elliptical machines and like the exercise, as I do.
The product field breaks down into two types: those with active arm motion and those with passive arm motion; these are my own designations.
There are only two active-arm-motion machines I'm aware of: StreetStrider and FreeCross. Both are lean-to-steer three-wheelers; you drive with all four limbs.
There's almost no information on the FreeCross, but it's made in Germany and appears to be built like a Mercedes; 20" Big Apples all around, a NuVinci N360 hub gear, and steering dampers. Weight ~60 lbs, rear fender standard, front fenders optional, folds for transport. No credit card sales; bank draft or PayPal is specified, but I contacted them and they will accept a cashier's check from a reputable bank. Price about $3000, shipping included.
The StreetStriders appear to be more lightly built but aren't; their frames are steel instead of aluminum; weight about 54 lbs. They've produced a lot of variations but now only offer two: a three-speed or seven-speed IGH with everything else identical. Go to their site and note the "Sale Ends ..." teaser at the top of the page ... it's the same sale, they just keep renaming it about every week. Meant to pull in a panic buy or two. Decent deal, though; they throw in an indoor trainer and free shipping; about $1000 for the 3-spd and ~$1300 for the 7-spd. They've gone cheap on a couple things; I think they should have built these with a steering damper, but the rear wheel is 20 x 1 3/8" or 37 x 451, and the selection of tires for it is very narrow.
The StreetStriders have a rear fender, but no option for front fenders. One of the things I cheer them for is that they offer luggage capability; no one else seems to have thought that might matter. They have a custom rear rack and I think two luggage pieces; they show a custom pannier. Don't fall for their Bundles without giving it some thought. I'd get the rack and fasten a milk-crate to it.
As to the passive-arm products, that market is dominated by Elliptigo. They off long-stride models (LS) with three choices of IGH; they have the Arc line, called 'compact-stride', and they offer a SUB, or Stand-Up-Bicycle. All are true bicycles with two wheels and handlebar steering.
I took short test rides on an Arc and a LS model. I was quite impressed at the quality of workmanship. I had problems with both, though, and dropped them from consideration. The LS is notorious for the noise it makes; it's like somebody swishing a shovel-blade through loose gravel. Also there was, to me, something subtly wrong about the way it felt; it was very smooth, but I thought it was weird and I didn't like it.
The Arc bothered me a lot because of a feature of its foot-platform; they're pivoted in the middle, and at the point in the pedal stroke where your weight shifts from front to back and vice-versa, the pedal flops over abruptly. No-go on that one, for me, also. I think they should find a way to buffer that motion somehow.
Aside from that, both the bikes I tried were very easy to get on and off of, very stable and smooth, very easy to handle. They put you way up in the air, so visibility is excellent. Elliptigo has a fanatical fan base, and I'm not surprised; they just weren't to my particular liking. They're expensive, too.
Just on the chance that you haven't tried anything at all elliptical, you should do that first. If you don't have membership in a fitness center, you can likely get a week's trial membership cheap. I'm still rebuilding my engine from years of disuse, and a cross-country program on the elliptical is very intense; I'm only good for 15 minutes so far before I turn to jelly, but I like it, and I like the idea of using all four limbs to power an outdoor ride.
Hope this helps.