I've been doing both in addition to cycling for the past 6 months while i recover from an injury.
The ellitpical is more like running and what i really like it for is watching the HR to make sure i'm hitting my normal easy day running HR. I can also do a killer speed session and get the HR up to Vo2 max levels. I've slowly been incorporating some running with the elliptical as i work my way back to full time running. But i decided that when i'm recovered, for easy days after speed workouts, i'm going to continue to use the elliptical for at least part of my workout. I can get my HR up to an "easy run" pace while the legs recover. When i run after a hard workout, the HR is barely going up.
the key to surviving the elliptical is a really good play list that you change up every day or you will bail in 20 minutes.
Swimming is so different biomechanicly from running but it's great for overall strength. It's also like running in a sense of having to hit paces for intervals so it's not quite as boring as the elliptical. But i wouldn't recommend it if you don't know all of the strokes and just swim laps. It's boring and you really won't get a good workout just floating around.
I've used long rides to take the place of long runs. But i also find that riding a 10 mile time trial on the bike is as hard if not harder than a 5k.
I'm hoping that by mixing all of these together, i'll be able to come back to running stronger.