Here's my two cents:
Ellipticals rock, I love the no-impact. But you will have to spend a LOT more (possibly double) for a really good quality elliptical than you would for a similar-quality treadmill. Go to Sears and/or your local sporting goods/exercise equipment store, and you will definitely find that $$$=quality. I actually went to a store near me called 2nd Wind Excercise (they are I think exclusive to the upper Midwest) and "test drove" a couple of floor models that the guy called "consumer-professional quality" - very smooth, very sturdy, and starting at $1,000. Then I went to Sears, Galyans (a sporting goods store) and a couple different consignment places, and none of the "department store brands (Nordic Track, Proform, Reebok) came even close in quality to the first ones I tried (which were Vision brand - and fabulous. I highly recommend them if you are looking to invest in a really high-quality machine -
www.visionfitness.com). I currently weigh 208 and even if the weight limit on the machine is 250 or 300, I don't want a machine that creaks like it's going to collapse when I step on it! So now I'm saving my pennies to eventually get the Vision one that I tried and loved. After experiencing a really good elliptical, I wouldn't take a lower-quality one if you offered it to me for free.
Here is some other advice that I have found through personal experience. There are LOTS and LOTS of ellipticals, mostly "hardly used," on ebay. BUT I would never buy one online without having tried a floor model first. Two models that look very similar in design can have a lot of differences in performance - the most important of which, I think, is the track of the pedals! Some models have a really short track (lots of knee action), others have a longer, flatter track. Some make you feel like you're tilting down into the floor, others are very level. Try some floor models and figure out what you prefer - down to a specific model if you can - and then you can search for that particular model online.
okay, definitely more than 2 cents - more like a buck and a half. But I hope it helps!
Fiona