You can buy a used one as long as you fully reset it before you start using it. Just Google it, it's a process of holding down a button, inserting a stick, doing a rain dance...

Takes probably about 30-60 seconds. You can't just turn it on and start it up, though. Either Ebay or Amazon, I forget which, sells the sticks for MUCH cheaper than you can get them at a drug store or Wal-Mart/Target/etc.
I have mine and I suppose I like it. I used it to get pregnant with Drew, but I was VERY bad at remembering to do it, so I can't credit it with my pregnancy. Right now it's giving me "High" readings (low, high, peak) and I'm not getting ANYTHING on OPKs, so I don't know what to think. *shrug* Guess it's good to have so much advance warning!
I think I like it though. It helps me feel proactive. It's considerably more expensive than just using OPKs, but it gives me a sense of security.