I spoke at AdTech yesterday. I heard two particularly interesting stats yesterday. Bill Tanner at Hitwise pointed out that 75% of search traffic comes from brand terms, and that advertising on your brand terms, beyond ranking #1 naturally, gives you 80+% of the traffic on those searches, while not advertising drops you under 60.
GarySmith, at YourAmigo.com, pointed out, on the other hand, they had a home decor etailer, and the top 100 phrases pulled only about 10% of their total traffic. I'm assuming their brand name wasn't particularly well known. This retailer had 32k out of 44k unique phrases get 1 click/month, and that the 3,4 or 5 word phrases had higher conversion rates. The long tail is alive and well, in this example :)