IMHO, online candidate search is a great service. I currently use two such websites and shell out almost $5000 per annum for the same. I always do a solid cost / benefit analysis and I have found that I am saving almost $15-$20K on my recruitment cost alone.
We manage to find decent candidates. However there are few limitations that you must keep in mind:
1) Candidates you get through online database search are more prone to attrition compared to those who are through a consultant. This happens because they have an active profile online and they continue to get offers from various companies.
2) You will get quality resumes for people with experience range of 0-8 years. People who are more qualified or experienced and hold important position in companies generally do not post their resumes to such sites.
3) Verification / validation of employees who come through these sites need to be done rigorously at your end. Like every online transaction, there is a probability of fraud in terms of forged resumes.
BTW, I found a very nice concept. Please check out http://www.yellojobs.com/ to see the new model of personal reference network for hiring. Its a mix between sites like linkedin.com and pure resume database sites like monster.com and naukri.com.
Abhishek