I love my treadmill. People will tell you that an eliptical is lower impact, and they're right, but the natural running motion of the treadmill does it for me. I'm actually on my second one as I recently killed my first (A POS NordicTrack), a fact I'm very proud of. I ran that fu@$#% into the ground!
That being said, a quality treadmill is an expensive piece of equipment. I spent $1700 and change on my new one (
Fitline T400) and that's really considered entry level for a good quality treadmill.
If you do decide to go with a treadmill, research it well and don't buy ProForm or the other Sports Authority/Modells/Walmart models - they will break within a year, as my NordicTrack did. Also, if you're planning on running on it, get a model with at least 2.0 horsepower
continuous duty, preferably 2.5 or even 3.0 HP (mine has a 3HP motor). Look for good warranties as well, with 3 years or more on the motor, and at least 1 year on parts/labor.
Good luck!