I have been composting with worms for over 8 years.
It is my opinion that is useless to add worms to correct soil.
The correct method is to correct your soil then the native worms will come.
You do not want to add foreign worms to your soil, regardless of the breed, it is a suicide mission and complete waste of your money.
You must first add tons (literally) of organic materials: compost, manure, leaf mold and such. THEN if the native worms are not appearing within 6-12 months you can add worms.
Regardless of the worms, you must keep a steady supply of organic materials to keep the worms in your yard. (compost, leaves, etc)

Just my 2 cents,
Liz
(owner of BigTex Worms)

Here is a link that might be useful: adding worms to lawn

The only advantage putting compost worms into the yard is the value of their decomposing bodies to the soil. I do not know what type of worms grow in soil other than it is the oposite of the kind that are needed to compost. If there are no worms in the soil then probably the soil is not right for worms. Just like the yard is probably not right for lions, thus there are no lions. The money would be better spent... not being spent. It is not money soil needs but life. Life is increased by adding decaying material to the soil and a variety of flora. Then there will not be anything you can do to keep the worms away. Buying worms for a garden is sort of like buying hormones for a teenager.

equinoxequinox, I'd never heard the phrase "buying hormones for a teenager." Priceless! :D