I can't address BB's, but here are my experiences with two other common products.
I've been running both Dynabeads and Centramics for about 2 years. Others have already provided the technical reasons why this works. I'll just provide daily driving experience, noting the pros and cons I’ve run across.
In short, at higher speeds (above 35-40) over longer distances with few stops these work well. Hence the reason they are used by over-the-road truckers. It should also be noted neither product will correct lateral imbalance.
Dynabeads -
http://www.innovativebalancing.com/
Pros
Flexibility, you can tailor the amount of product used to your tire size etc.
The larger the tire (within reason) the better they tend to work with the proper amount of product.
No wheel removal needed. Simply remove air from tire, pull the inner valve core and feed into tire
Cons
Does not work until speed increase above (roughly) 20 MPH. If your driving consists of primarily city driving (slow speeds with stop and goes) effectiveness will be greatly diminished.
On some installs you can hear a slight rumble of the beads until you get up to (roughly) 20 MPH.
Care must be taken when changing tires, otherwise you have beads all over the place.
Can be a pain to install within mounted tires.
At slower speeds hitting bumps can dislodge material resulting in temporary loss of balance.
Airing-down can be problematic as beads can cause issues with this.
I don't believe they can be used with run flat tires
Centramatic -
http://www.centramatic.com/
Pros
Fast install on mounted tires, simply remove wheel, slap on to hub reinstall wheel.
No issue when changing tires, plates stay mounted on hubs as tires are changed.
Much less prone to slower speed bump issues that effect Dynabeads
Can be used with run flat tires
Cons
Becomes less effective as tire size grows.
You can't increase/decrease balancing media to tire size
Wheel bolt pattern must be know when ordering.
Must remove wheels to place on hubs
Reduces brake air cooling
Diameter sizing can cause interfere with rim and/or caliper clearance.
Does not work until speed increase above (roughly) 20 MPH. If your driving consists is primarily city driving (slow speeds with stop and goes) effectiveness will be greatly diminished.
On some installs you can hear a slight rumble of the beads until you get up to (roughly) 20 MPH.
I've tried these products both separately and together on the same vehicle with same tires. My personal belief is Centramatics becomes ineffective above 31's. On my 35's they appeared worthless.
On a small car, they might work very well.
Hope this helps...