Since FDR's selection of Harry S. Truman as Vice-president in 1944. exactly 45.4%, or a little under half, of a president's choice of his deputy has gone on to become president. I arrive at that number by counting the total number of presidents elected in their own right from Franklin D. Roosevelt to present (11), counting the number of vice-presidents serving under those eleven men who went on to become president (5), and dividing the vice-presidents-turned-presidents by the total presidential administrations counted (11).
Further, almost thirty percent (27.2%) of those vice-presidents who became president themselves did so by law (when the president died in office or was assassinated or resigned): FDR/Truman, Kennedy/Johnson, Nixon/Ford. It is also worthwhile to point out that both Ford and Reagan were targets of assassination threats where actual bullets were fired: although Ford escaped unharmed, Reagan was wounded.
Considering that record combined with the fact that John McCain at 72 years shall be the oldest elected president by four years (in other words, an entire presidential term), it seems his choice of a novice, Palin, is reckless and irresponsible: exactly the type of choice America cannot afford given the current domestic and international crises facing the United States.
Moreover, Palin is under investigation by Alaska state authorities for allegedly seeking to fire a former brother-in-law from the state police (ala US attorney firings at the US Justice Department under the current Bush Administration) and is considered a hardline ideologue with little appetite for compromise.
The vice-president's duties under the United States Constitution are to succeed the president upon incapacity and to preside over the US Senate. Palin is not qualified to be president and is too ideological to preside over the US Senate: the world's greatest deliberative body.
All said, an irresponsible choice by McCain.