Martha,
A couple of comments.
- My Background: I swim anywhere from once to four times a week. I'm a long way from being an Olympic quality swimmer but I do swim with a Master's group and do occasional triathlons so I'm a fairly strong swimmer compared to the average guy.
- Most of the "serious swimmers" that I know have Zoomers or something similar.
- I would not recommend the very long, very flexible type fins that divers use.
- The Zoomers that you show come in two types "Red" and "Blue." I think that the Blue is best for most people. The Red is for sprinters. (I may have my colors backwards so verify this.)
- They also have another type that is slightly longer and slightly more flexible called the "Z2." They are blue with white tips. That is what I currently have.
- In the past I had some Blue type Zoomers. I can't really say I liked one type more than the other. I'm fine with both of them. The Z2s are a little faster.
- I don't use fins when swimming very often but I use them almost 100% of the time when I'm just kicking. I dislike kicking without fins. Fins make it more fun because you can go a lot faster.
- I think that the above comment probably applies to most beginning swimmers for swimming as well as kicking. With fins you may be able to swim farther and faster, enjoy it more and get more exercise without getting as tired.
- I would alternate with and without fins just because I would want to become a better swimmer without them but that's just my preference. Some people use them all of the time.
- The Zoomers company advertises that you can get a better aerobic workout with Zoomers vs straight swimming and I suspect that is true for many swimmers.
- I have read that stiffer fins can put more stress on your knees but I have never had any problems with it despite some pretty bad knee injuries from other sports and I don't know of any one that has had problems. In fact I have used them to help rehabilitate from knee injuries. Some fins are made to be less stiff or have slits or holes that are supposed to help minimize stress. I suggest looking into this if you have knee problems otherwise I don't think it is an issue.
- If you swim consistently I recommend getting some. They only cost about $30 and I think that they are worth the investment.