As endorsement signings go, I think this will be bigger for Callaway than signing Rory McIlroy will be for Nike. By that I mean, impact at the cash register.
Yes, Ishikawa will produce more business for Callaway than McIlroy does for Nike. Why? Asia.
What about the American market? No contest there: It's McIlroy. But Callaway grabbed Ishikawa away from Yonex for Asia, not America. (Although it's certainly possible that Ishikawa will impact the American market, too. He's playing on the PGA Tour in 2013. And I am convinced that he will become, probaby this year, an impact player outside of Japan.)
An interesting question is which Callaway equipment Ishikawa will play. Most golf fans probably don't know this, but Callaway sells equipment in Japan that it doesn't put on the market in the United States.
Callaway's press release states that Ishikawa will play the RAZR Fit Xtreme driver and Odyssey putters, but doesn't specifically mention irons or wedges.
Could that be because Ishikawa will be playing the Callaway Legend irons that are sold in Japan but not the US? The Legend irons are very highly regarded by golf gearheads who've seen them; they are considered higher-quality than the irons Callaway sells in the U.S. So why aren't they sold in America?
Price. The Asian golf market tolerates - expects, really - top-quality, high-priced golf clubs. Companies that have traditionally ruled the Asian markets, such as Honma and Miura, make expensive, beautiful sets of irons. Irons that are considered luxury items in the American market, where price is more of a deciding factor with consumers.
The Callaway Legend irons sold in Japan can run $2,000 per set. That just won't fly at Dick's Sporting Goods or Academy Sports & Outdoors.