I purchased one rather reasonably online. My PT suggested that I get one and if I do nothing else at home now that I have been released from PT, I should use it every day. It beats buying an exercise bike or having to find a gym somewhere. I can also use it table top if need be in the future. I say go for it. I think I got mine for around $20.00 plus shipping. Nothing fancy, just the exerciser.
I have one, my PT recommended it and also got it for me. She got it from WalMart for around $22.00. I've added it to my routine and use it twice a day. Figured it can't hurt, since I was using her's during PT. My therory is the more movement for the knee the better......at least I hope.
If you get an exercise bike, be sure the seat is comfortable. I rode mine Saturday night for the first time (I have had it for several years but was unable to ride it), for 15 minutes and the friction really hurt where you sit on the seat. I am going to try to find one of those gel seats designed for bicycles and if it eases the pain I will definitely let you know!
Based on your comments, I ordered a pedal exerciser yesterday. I hope it will be helpful. I use a medicine ball to roll back and forth to work on ROM, but I think the pedalling will be even better. Thanks for the information!