The other search engines do get some considerable traffic. Is it worth investing time/money in them as well?
If you've got the time and money, yes.
If a diverse range of traffic is required, yes.
I know people who hate Google and refuse to use it, ever. There's still a market for other websites out there. Another thing to consider however is that fact that over time as general traffic increases, search engines other than Google should have no trouble finding you... at least the top few search engines anyway.
You may want to think about smaller PPC companies like 7 search, etc. They are a great way to test ads for very low cost per clicks. Once you have an Ad that is fine tuned you can move it over to one of the larger engines.
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I would say yes for two reasons:
Google does not have 100% of the market.
The competition on Google is stiff, while with the other search engines
you can easily dominate them.
As far as PPC is concerned, you can still get clicks from $0.01 if you know what you are
doing on the other networks, not on Google from my experience!