Flippers?

As with any question of this type you'll get some who use them and love them, and others who wouldn't be caught dead using them.

As far as improving your swimming technique they do not provide much assistance, and actually can help you develop poor swimming habits - body positioning is especially affected.

I do find that they are a good tool for improving ankle flexibility (which helps with running) and they provide a nice recovery workout following long runs or bike rides.

They are also useful for drills especially if you have a weak kick

I posted a response to this question a while ago, and the put it on my website:

http://www.steelcityendurance.com/general/paddles-and-fins-.should-...

Note that it actually takes more energy to move the larger surface area of a fin through the water than to move your foot through the water. So your forward speed is accomplished with a slower frequency of kicking and less amplitude of kicking. Trying to maintain your usual kick frequency & amplitude will tire you out really quickly.

Sometimes, though, they are just plain fun to use.