Nielsen/NetRatings, released recent market share data:
Ask Jeeves: 6%
All the other SE's combined have approximately a 1.5% market share.
But, don't take that into consideration alone. Place tracking code on your URL's and webpages, and track. Test, Analyze the data. You might find that enhance interactive is the best converting SE, and not Google. You might get the best ROI on MIVA. Who really knows? It's different in each industry. BigResearch just conducted a study and found that in the Apparel industry the best converting SE's are in this order:
5. Ask Jeeves
So, if you are just interested in traffic volume, then Google is the answer. But, if you want to know which converts the best, then test and analyze for your individual results.
MSN has always produced the best audience for me. Surfers arriving from MSN stay the longest and visit the most number of pages in a session. But they just are simply not capable of delivering anywhere near the amount of visitors as Google is. And their PPC campaigns are way more expensive per click too! Unfortunately, even with Google's "lesser-quality visitors" as far as my specific traffic is concerned, it just doesn't balance out, and I have to stick to mostly organic with a little dash of AdWords thrown in.
I keep hearing that people have better results with the smaller search engines. You can bid less, and for some reason they convert better, resulting in a drastically improved ROI. However, for me, I cannot validate the time and effort required to set up a PPC campaign to reach their miniscule 1% market share.