It depends on who you talk to about this. Remember riding the single speed bicycle as a child and wishing for gears and hand brakes? Now do this on an adult size bicycle. The fixie will let you pedal backwards, though.
I believe that using the legs to stop and slow down the bicycle will put stress on the knees. There are good and bad points to consider as well as where this bicycle will be ridden: road, track or trails, on hills or level surfaces.
There is a lot to consider before making a purchase; therefore, I strongly suggest renting one for a day.
I will stick with my bike with gears and hand brakes. When I want a good workout to gain strength, I keep the bicycle on the hardest gear. The options make good safety sense to me, as well.
Enjoy your biking experiences.