From most of the scouting stuff I've read, Tanaka has a wipeout splitter and excellent command. The big risk is his workload at his age - and whether it portends something problematic. The other risk is whether his approach is too "Japanese". Darvish's approach was (even in WBC evidence) very American - pitch to contact, come right at hitters with your best stuff. Daisuke never made that adjustment. Having 6 pitches and nibbling works in Japan - here you are better off throwing your 3 best pitches with location and command and going after guys. Dice-K lacked either the confidence, or was just too stubborn. I lean to the latter since he flashed evidence of the tools, and still does.
That said, we know based on the current salaries and the way you expect them to inflate that a B/B+ starter who can provide a lot of bulk is going to be worth 20-25 million. So the dollars for Tanaka, especially given the Yankees balance sheet - is fair. There is a risk, but that describes all deals for pitchers, including ones in this offseason's lot (like Ubaldo Jimenez). The gamble on a 25 year old makes more sense. My guess, just based on reading and not having seen him, is his ceiling is more or less Jon Lester - not one of the top 5 pitchers in the league, but capable of touching that level from time to time, but able to churn out 200 really solid innings without any real drama.
Off Topic - Edes' speculation of Lester's extension being 5/120 or so seems totally reasonable based on all the comps ...