I agree with SpaceManSpiff. Better than cycling, not
as good as (but mentally easier than) pool running. You
can definitely sustain a higher heart rate with less
effort on the elliptical than on either the bike or in
the pool.
Watch out for the wide foot placement - for men in
particular most ellipticals have the feet too far apart.
I tend to feel it in my adductors after a long workout.
It helps to change the incline slightly every so often
(but keep it neer level and keep the cadence high). Try
to stay loose - don't get a death grip on the rails or
your back will get tight. I also find it helps to move
my feet around on the pedals every now and then.
The elliptical is great for training around some injuries
(zero impact), but for others it can be as stressful as
running, so you have to be careful. Build up your time
on the elliptical gradually - if you are in good running
shape it transfers easily to the elliptical, so you can
go a long time without a lot of effort, but you will likely
regret it if you just hop on and go for two hours the first
time you do it.