I bought one last year and I love it. I did a fair bit of research before I bought it, and there are few things to consider:
Generally you get what you pay for. If it's cheap, it's because it was made that way and will be much more likely to break down. The more bells and whistles a machine has, the less it tends to be focused on basic quality. If you can't afford much, check out places like Craigslist for a better model used and cheaper (there's usually a bunch of them hitting the for sale section arounf February or March - people buy them as New Years resolution machines and then never follow through).
Weight capacity. If you are near or at the max weight capacity of an elliptical, it's probably better to look at one with a higher one max weight capacity, or use one at a gym until you have reached a safer weight. This isn't just a worry that you can put too much wear on the machine or break it, but that you can tip it over, and injure yourself.
The heavier the flywheel, the better. 14 pounds is a good low end, and heavier is better. Flywheels come positioned at the front or at the back. The ellipticals with a flywheel at the front tend to take up slightly less floor space, but they're still BIG.
Stride length is very important. I have very long legs and I am 5'8" tall, and I find that our elliptical's stride length of 18" to be barely long enough. If a tall guy is going to use it, it will do, but 20" is better. If you are average or short in height, you might be happy with a 16", but I don't recommend it. 18" is a minimum good length.
Here (treadmill sensei)
is a good place to take a look at what is available, and why some models are better than others. He gives The Nordic Track ellipticals have a bad repair record, and other review places I have looked at agreed. I know some people who have bought Nordic Tracks and are happy with them, but I also know a couple of people who have thrown theirs in the trash.
I bought a Horizon EX 66 because, of the highly rated ones in my price range, it had the smallest footprint, and that was a major factor because I live in an appartment. I would have seriously considered a Schwinn 431, but it was out of stock at the time, and I would have loved a Sole E25, but that was too big. The Kettler CT2000 has too small a stride for me, so it was right out.
I found it took only a week for me to get up to 20 minute workouts, and it took my DH only 3 days. It's an interesting motion, kind of like a combination of running on a stairmaster. You get used to it quickly. They are a cardio machine, so you can get a real workout on one, but you can go as slow or as fast as you want. They have different levels of resistance, so you can start off easy, and increase as you feel ready. They are pretty kick-butt for burning calories once you really get going - probably not quite as good as running, but they have that nice no impact bit.