I use both Voice 123 and Voices.com and have had success with both. The free membership allows you to post your demo and hope that a producer will find you among all the others. With the paid membership, however, you are sent plenty of auditions and have many opportunities to compete for a job. Paid is really the way to go.
I've found that I do best with these services when I really work them - that is, do as many auditions as my schedule allows. I generally have to do 30-40 auditions for each job I book. I've gotten gigs from these services that paid as little as $100, and as much as $2000.
Some jobs are just 30 second radio or TV spots, others are narrations that run several minutes. Most producers in search of a voice are looking for just a VO - they usually don't ask for a fully-produced spot or narration.
Remember that the people who hire you are your customers, and if you provide excellent customer service they'll come back again and again.
If you have the talent and if you invest the time to do lots of auditions, you will earn far more than the cost of membership.
Nick Summers