Obviously every site will vary in terms of traffic, how much it receives, where it comes from, how popular it is, etc., etc.
Here in this example screen shot from my Google Analytics it very easy to see how much traffic Google alone can bring or generate.
For those of you that believe Yahoo, Bing, or any other search engine is important and does a good job, I would recheck your stats.
If you don't get much traffic than it might not be as apparent. If you are only getting a couple of visitors per day, it will be more difficult to see this.
Here in this screen shot you can easily see Google out performs them all considerably.
These stats are actually for this site from my own Google Analytics account. The time frame is irrelevant and doesn't really matter. What does matter are the numbers.
As you can see for this site Yahoo and Bing/MSN combined doesn't even come close to Google.
So even if you do not use or like Google, it is important to learn about the things Google likes. The more Google likes your site the more traffic you can expect to see.
If Yahoo and Bing/MSN sent more traffic I would spend more time with them. I do a little optimizing for them in my spare time when I get a chance, just not nearly as much as I do for Google.
Right now and for a number of years Google is the best traffic source.