I purchased a ProForm from Sears 13 months ago and absolutely love it! It only cost me $800. I use it 5-6 days a week and rarely for less than 40 minutes...usually an hour.
Look for a nice wide belt. Consider who will be using it, how often and their weight. As those factors increase, the tougher the machine you need. Both DH and I are small so our machine doesn't take the beating that it would if we were larger, therefore we can get away with an $800 machine.
There are a lot of people here who have ProForms and have had them 10+ years.