In general, I'm not a huge fan of pedal exercisers, but especially the low cost ones. They just are not very stable, which makes them quite jerky when you use them. Look for heavier ones with a nice wide base to overcome this tendency.
Note that there are basically two types of pedal exercisers. One (like the one you show in the photo), is supposed to mimic an elliptical machine. The other type mimics an exercise bike.
I'm not a fan of ellipticals at all, but especially for someone rehabbing from a knee replacement, I believe they should be avoided. The elliptical motion itself is not a natural movement, and it forces you to over engage your quadriceps muscles. This can put extra stress on your quadriceps tendon and patella and cause pain. This also can create muscular imbalance (overworking your quads and not your glutes or hamstrings), leaving you more likely to injure yourself. If you are determined to get this type of machine anyway, at least make sure it also allows you to go in reverse as this will help address the shortcomings above.
It probably should be clear by now, but I personally think a better choice would be a traditional "bike-like" pedal exerciser. This is a more natural motion.
I should note that If you're going to Physical Therapy post surgery (and hopefully pre-surgery as well), then you may not need to buy anything. Once you're through with PT, you should be fine walking. Walking is the best exercise you can do post surgery, since it's what our bodies were meant to do.
If you do decide you need one, DeskCycle is probably the best known brand and likely the only one I'd personally buy if I were shopping for one.
Just my thoughts and not meant to dis anyone that loves their elliptical.