Pull buoys are useful primarily because they reduce the amount of large, sweeping kicks that swimmers usually make -- in short, they encourage faster development of upper body strength. They also help you focus your mind on arm motion and breathing.
Pull buoys are very useful in training situations. However, it's worth noting that not all coaches and swimmers advise the frequent use of buoys, as they feel the buoys interfere with proper technique and create sloppy strokes.
Of course, to get any benefit at all from these products, you have to have the right buoy. It must fit your body and be comfortable as you swim. You should be able to backstroke, flip turn and push off with the buoy in place.
With the right product, you can gain a better sense of balance in the water, as well as increased upper body strength. Moderation is key. You may want to add just a single set of buoyed laps to your normal routine.
Because you're omitting half of your stroke (the kick), a buoy frees you to really concentrate on mastering your arm-stroke technique. Take advantage of this and really work out the kinks in your freestyle stroke.
Buoys are also great because they eliminate kicking. Kicking is hard work that leaves even experienced swimmers gasping. Minus the kicking, your breathing will be more relaxed, letting you work on disciplined breathing.
If you generally swim at a slower pace, you will find that your freestyle stroke feels easier with a buoy. Thus, at this easy pace, buoys are a great way to work on the technique of your stroke, building good swimming habits that carry over to buoy-less outings.
You'll feel your body glide easily through the water as you use the buoy. You'll be even more hydrodynamic if you point your toes during your workout. But be sure not to clench your legs or your toes -- this can lead to cramps.
With a bit of practice, you'll feel at ease working out with a pull buoy. After just a couple of weeks of consistent use, your upper body will begin to feel stronger, and with some attention to detail, your strokes will be more precise and powerful, too.