Effectiveness in this order:
Nepotism -> experience -> qualifications
About 75-80% of the time, the first one will be what gets you a job.
What industry are you referring to?
I guess if you're talking about small businesses then perhaps but for most large companies you'd need to know someone very high up to bypass HR.
For banking even knowing an MD* is likely not enough these days - sure they can give you a contact who's hiring or maybe help you get an interview but the days where they could simply get you a job are long gone.
*its not as high up as you'd expect mind - this is an industry where people can make it to 'Vice President' in their 20s...