Bearing in mind that this is still in the territory of "leaked before actually announced", and as much as they're now closer to San Jose than they are to Oakland (at least both in my memory and on the map, pending a dictatorial ruling), but the bottom line simply seems to be that this was the best real estate deal they could grind out.
But in the long term it could work well for them if it means they can draw more of the San Jose populous than they are already, plus the fact that it'll be a shiny new balpark, which usually draws well the first few seasons.
Baseball Prospectus lists Clemens at the top, with a VORP (Value Over Replacement Pitcher) of 49.8. What that means is, Roger Clemens as a pitcher kept 50 runs from scoring more than an average pitcher would do in his place. Willis is second with 46.