Last winter I did the elliptical for a few weeks. I would wait until I could get on a Cybex "Arc Trainer". On other machines I felt like I was about to run right out of the little platforms that you put your feet in. The resistance and area of travel on the Arc Trainer seems more natural. The other machines felt very vertical. It felt like the upstroke of the pedal was going to launch me right out of the machine.
Anyway I'd set it to 7 (out of 10, or 12, I can't remember), set it to "fat burning" mode, and go for 50 minutes or 1000 calories whichever came first. Of course you don't put much stock in the calorie count.
After 15 minutes the sweat would be pouring off me and the neighboring customers would be getting sprayed. I can't do that on an exercise bike. I never really paid much attention to my HR; just "perceived effort". The "fat burning" mode on this machine gives you hard efforts and recoveries in a ten minute cycle, if I remember right. The hard efforts would be two minutes long and were hard enough for me that I'd be really looking forward to the recovery part of the program.
So I think they're OK for maintaining some semblance of aerobic fitness. As long as (as with anything else) you do it consistently, of course. Way better than a bike for me. Pool running, I'd be out of my mind within ten minutes.
Walk around carefully for a few minutes each time you get off the elliptical, and check out how your knees are doing. I think these machines will tend to tweak your knees with the repetitive motion, especially if you tend to really stand on the pedals hard on each stroke.